fix(agents): make librarian failures structured and user-safe

- run_librarian / run_librarian_stream raise AgentError instead of
  returning/yielding error text as normal output; detail stays in logs
- delegate_to_* wrappers now put a curated butler-toned sentence in
  DelegationResult.output on failure and never expose str(e), so
  streaming's error branch is reachable and honest
- _execute_single_delegation propagates success; direct delegation only
  records delegate_to_* as called when the expert actually succeeded
- librarian tools return user-safe messages instead of
  'Error searching: {e}' strings that leaked internal URLs into
  synthesis; coordination stream errors are curated as well
- ruff cleanups (TYPE_CHECKING forward refs, B904, unused locals) in
  the touched files to keep them lint-clean

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/agents/test_delegation.py

@@ -4,18 +4,19 @@ Tests for delegation infrastructure.
 Tests the DelegationTask dataclass and delegation wrapper functions
 that implement the agent-as-tool pattern.
 """
+from unittest.mock import AsyncMock, patch
+
 import pytest
-from unittest.mock import AsyncMock, patch, MagicMock
 
 from src.agents.delegation import (
-    ActionType,
-    DelegationTask,
-    DelegationResult,
     HOUSEHOLD_THINK_MESSAGES,
     STREAMING_DELEGATION_WRAPPERS,
+    ActionType,
+    DelegationResult,
+    DelegationTask,
+    _detect_action_type,
     delegate_to_librarian,
     get_think_message,
-    _detect_action_type,
 )
 
 
@@ -170,11 +171,11 @@ class TestDelegateToLibrarian:
 
     @pytest.mark.asyncio
     async def test_delegate_to_librarian_handles_error(self):
-        """Test delegation handles Librarian errors gracefully."""
+        """Test delegation maps Librarian errors to a user-safe result."""
         with patch(
             "src.agents.librarian.agent.run_librarian",
             new_callable=AsyncMock,
-            side_effect=Exception("Connection refused"),
+            side_effect=Exception("Connection refused to http://internal:8089"),
         ):
             result = await delegate_to_librarian(
                 task="Search for information",
@@ -182,8 +183,15 @@ class TestDelegateToLibrarian:
 
             assert isinstance(result, DelegationResult)
             assert result.success is False
-            assert result.output == ""
-            assert result.error == "Connection refused"
+            # Output carries a curated butler-toned sentence
+            assert result.output == get_think_message(
+                "librarian", "Search for information", "error"
+            )
+            # Exception detail stays in logs only - never in the result
+            assert "Connection refused" not in result.output
+            assert result.error is not None
+            assert "Connection refused" not in result.error
+            assert "internal" not in result.error
 
     @pytest.mark.asyncio
     async def test_delegate_to_librarian_preserves_task(self):