fix(tests): correct two independent AttributeErrors in test_task_executor

1. `executor.max_concurrent` has never existed. Concurrency is capped by the
   module-level MAX_CONCURRENT_TASKS constant via asyncio.Semaphore(
   MAX_CONCURRENT_TASKS) in TaskExecutor.__init__ — confirmed with git log -p
   across this file's whole history (three commits), the name has always
   been the module constant, never an instance attribute.
   test_executor_initialization now asserts MAX_CONCURRENT_TASKS == 5 and the
   semaphore's initial count, instead of a name the class never had.
   test_concurrent_task_limit asserted `mock_execute.call_count <=
   executor.max_concurrent`, which — separately from the AttributeError — was
   asserting the wrong observable: process_minute() awaits the full batch via
   asyncio.gather before returning, so by the time the assertion runs all 10
   scheduled tasks have executed; the semaphore bounds how many run
   concurrently mid-flight, not the eventual call_count. Reworded to assert
   all scheduled tasks still run (call_count == len(tasks)); a concurrency-
   in-flight assertion would need a task that can be observed mid-execution,
   which the AsyncMock stand-in does not provide.

2. _run_executor (src/tasks/executor.py) loads the executor module with the
   __import__ builtin directly (`__import__(module_path, fromlist=
   ['execute'])`), not importlib.import_module — this repo's own CLAUDE.md
   documents it as "the thing that will mislead you" about this module.
   importlib is never imported there, so patch('src.tasks.executor.importlib.
   import_module') failed at patch setup, before the three
   test_execute_task_* bodies ran at all. Switched to patch('builtins.
   __import__', side_effect=...) with a routing function that falls through
   to the real import for anything other than the target module — verified
   the call-recording shape empirically first (call('name', fromlist=[...])).

Source is unchanged in both cases; both are test-only defects present since
this file's initial commit.

tests/test_task_executor.py

@@ -4,7 +4,7 @@ Tests for the task executor module.
 import pytest
 from unittest.mock import AsyncMock, MagicMock, patch
 from datetime import datetime
-from src.tasks.executor import TaskExecutor
+from src.tasks.executor import TaskExecutor, MAX_CONCURRENT_TASKS
 from src.config import Settings
 
 
@@ -17,7 +17,13 @@ class TestTaskExecutor:
         executor = TaskExecutor(test_settings)
 
         assert executor.settings == test_settings
-        assert executor.max_concurrent == 5
+        # There is no `max_concurrent` instance attribute — concurrency is
+        # capped by MAX_CONCURRENT_TASKS (module constant) via
+        # asyncio.Semaphore(MAX_CONCURRENT_TASKS) in __init__. Verify the
+        # semaphore was built with that bound instead of asserting an
+        # attribute name the class has never had.
+        assert MAX_CONCURRENT_TASKS == 5
+        assert executor.semaphore._value == 5
 
     @patch('psycopg2.connect')
     def test_get_db_connection(self, mock_connect, test_settings: Settings):
@@ -87,13 +93,25 @@ class TestTaskExecutor:
 
         task = {**sample_task_data, 'id': 1}
 
-        with patch.object(executor, 'get_db_connection') as mock_get_conn, \
-             patch('src.tasks.executor.importlib.import_module') as mock_import:
+        # _run_executor (src/tasks/executor.py) loads the executor module with
+        # the __import__ builtin directly — `module = __import__(module_path,
+        # fromlist=['execute'])` — not importlib.import_module. This is the
+        # documented dynamic-loading trap in this repo's own CLAUDE.md
+        # ("executors are chosen by data, not code"). `importlib` is never
+        # imported in that module, so patching 'src.tasks.executor.importlib'
+        # fails at patch setup, before the test body runs at all.
+        real_import = __import__
 
-            # Mock executor module
-            mock_executor_module = MagicMock()
-            mock_executor_module.execute = AsyncMock(return_value="Task completed successfully")
-            mock_import.return_value = mock_executor_module
+        def fake_import(name, globals=None, locals=None, fromlist=(), level=0):
+            if name == 'src.executors.example_executor':
+                return mock_executor_module
+            return real_import(name, globals, locals, fromlist, level)
+
+        mock_executor_module = MagicMock()
+        mock_executor_module.execute = AsyncMock(return_value="Task completed successfully")
+
+        with patch.object(executor, 'get_db_connection') as mock_get_conn, \
+             patch('builtins.__import__', side_effect=fake_import) as mock_import:
 
             # Mock database
             mock_conn = MagicMock()
@@ -106,7 +124,7 @@ class TestTaskExecutor:
             await executor.execute_task(task)
 
             # Should have imported executor module
-            mock_import.assert_called_with('src.executors.example_executor')
+            mock_import.assert_any_call('src.executors.example_executor', fromlist=['execute'])
 
             # Should have updated task status
             assert mock_cursor.execute.call_count >= 2  # Insert execution record + update task
@@ -118,13 +136,20 @@ class TestTaskExecutor:
 
         task = {**sample_task_data, 'id': 1}
 
-        with patch.object(executor, 'get_db_connection') as mock_get_conn, \
-             patch('src.tasks.executor.importlib.import_module') as mock_import:
+        # See test_execute_task_success: _run_executor uses the __import__
+        # builtin directly, not importlib.import_module.
+        real_import = __import__
 
-            # Mock executor that raises error
-            mock_executor_module = MagicMock()
-            mock_executor_module.execute = AsyncMock(side_effect=Exception("Task failed"))
-            mock_import.return_value = mock_executor_module
+        def fake_import(name, globals=None, locals=None, fromlist=(), level=0):
+            if name == 'src.executors.example_executor':
+                return mock_executor_module
+            return real_import(name, globals, locals, fromlist, level)
+
+        mock_executor_module = MagicMock()
+        mock_executor_module.execute = AsyncMock(side_effect=Exception("Task failed"))
+
+        with patch.object(executor, 'get_db_connection') as mock_get_conn, \
+             patch('builtins.__import__', side_effect=fake_import):
 
             # Mock database
             mock_conn = MagicMock()
@@ -149,19 +174,26 @@ class TestTaskExecutor:
 
         task = {**sample_task_data, 'id': 1, 'timeout_seconds': 1}
 
+        # See test_execute_task_success: _run_executor uses the __import__
+        # builtin directly, not importlib.import_module.
+        import asyncio
+        real_import = __import__
+
+        def fake_import(name, globals=None, locals=None, fromlist=(), level=0):
+            if name == 'src.executors.example_executor':
+                return mock_executor_module
+            return real_import(name, globals, locals, fromlist, level)
+
+        mock_executor_module = MagicMock()
+
+        async def slow_execute(*args, **kwargs):
+            await asyncio.sleep(10)  # Longer than timeout
+            return "Done"
+
+        mock_executor_module.execute = slow_execute
+
         with patch.object(executor, 'get_db_connection') as mock_get_conn, \
-             patch('src.tasks.executor.importlib.import_module') as mock_import:
-
-            # Mock executor that takes too long
-            import asyncio
-            mock_executor_module = MagicMock()
-
-            async def slow_execute(*args, **kwargs):
-                await asyncio.sleep(10)  # Longer than timeout
-                return "Done"
-
-            mock_executor_module.execute = slow_execute
-            mock_import.return_value = mock_executor_module
+             patch('builtins.__import__', side_effect=fake_import):
 
             # Mock database
             mock_conn = MagicMock()
@@ -303,8 +335,16 @@ class TestTaskExecutor:
 
             await executor.process_minute()
 
-            # Should only execute max_concurrent (5) tasks
-            assert mock_execute.call_count <= executor.max_concurrent
+            # process_minute awaits every task in this batch (asyncio.gather)
+            # before returning, so by the time we're back here all 10 have
+            # run to completion — the semaphore bounds how many can be
+            # in flight *concurrently*, not the eventual call_count, which
+            # this assertion conflated. Kept as a correctness check on the
+            # total (all scheduled tasks still get executed) since a
+            # concurrency-in-flight assertion needs a task that can be
+            # observed mid-execution, which mock_execute (an AsyncMock with
+            # no delay) does not provide.
+            assert mock_execute.call_count == len(tasks)
 
 
 @pytest.mark.unit