style: apply ruff's automatic fixes and formatter

Mechanical only, and separated from the judgment calls that follow so the
reviewable changes are not buried in a 98-file whitespace diff.

227 automatic fixes: 60 blank lines carrying whitespace, 60 unsorted import
blocks, 34 Optional[X] to X | None, 28 unused imports, 16 deprecated typing
imports, 12 datetime.timezone.utc to datetime.UTC, and assorted smaller
modernisations. Then `ruff format` over src and tests: 98 files reformatted,
35 already conforming.

No file among the unused-import findings defines __all__ or is an __init__.py,
so nothing here removes a re-export.

`make test`: 658 passed, unchanged from HEAD.

Two things observed while verifying, neither addressed here:

`pytest tests/` cannot collect — tests/e2e/test_orchestration_e2e.py uses an
`e2e` marker that is not registered, and the config is strict about markers.
This fails identically at HEAD, so it predates this change; `make test` passes
because it ignores tests/e2e, tests/integration and tests/contracts.

test_tatlock_tool_call_logging_calculator is flaky. It failed once in a full run
with these changes and passed on the next, passes in isolation with them, and
fails in isolation at HEAD. It is order- or timing-dependent, not a regression
from this commit — established by running the full suite both ways rather than
by reasoning about which change could have caused it.

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

tests/conftest.py

@@ -2,11 +2,12 @@
 Shared test fixtures for all tests.
 Following FastAPI testing best practices.
 """
+
 import asyncio
 
 import pytest
 from fastapi.testclient import TestClient
-from httpx import AsyncClient, ASGITransport
+from httpx import ASGITransport, AsyncClient
 
 from src.main import app
 
@@ -28,10 +29,7 @@ def _tenant_guard():
     from src.core.multi_tenancy import get_memory_collection_name
 
     effective = get_default_user()
-    if (
-        effective == PRODUCTION_TENANT
-        or config.effective_default_user == PRODUCTION_TENANT
-    ):
+    if effective == PRODUCTION_TENANT or config.effective_default_user == PRODUCTION_TENANT:
         pytest.exit(
             f"TENANT GUARD: refusing to run the test suite - the effective "
             f"tenant resolves to the production tenant '{PRODUCTION_TENANT}' "
@@ -59,6 +57,7 @@ def _initialize_app(_tenant_guard):
     production tenant before any initialization happens.
     """
     from src.core.startup import initialize_application
+
     asyncio.run(initialize_application())
 
 
@@ -79,10 +78,7 @@ async def async_client() -> AsyncClient:
 
     Use for testing async endpoints and streaming.
     """
-    async with AsyncClient(
-        transport=ASGITransport(app=app),
-        base_url="http://test"
-    ) as client:
+    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as client:
         yield client
 
 
@@ -91,9 +87,7 @@ def mock_chat_request() -> dict:
     """Standard chat completion request fixture."""
     return {
         "model": "lorem-tester",
-        "messages": [
-            {"role": "user", "content": "Hello, world!"}
-        ],
+        "messages": [{"role": "user", "content": "Hello, world!"}],
         "temperature": 0.7,
         "stream": False,
     }