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>

src/responses/router.py

@@ -7,10 +7,10 @@ OpenAI-compatible /v1/responses endpoint with streaming support.
 from fastapi import APIRouter, HTTPException
 from sse_starlette.sse import EventSourceResponse
 
-from src.responses import service
-from src.responses.schemas import ResponseRequest, Response
-from src.core.exceptions import ModelNotFoundError, AppException
+from src.core.exceptions import AppException, ModelNotFoundError
 from src.core.logging_config import get_logger
+from src.responses import service
+from src.responses.schemas import Response, ResponseRequest
 
 logger = get_logger(__name__)
 
@@ -58,14 +58,11 @@ async def create_response(
             if use_steward:
                 logger.info("Streaming with Steward preprocessing for Tatlock request")
                 from src.responses.streaming import StreamingCoordinator
+
                 coordinator = StreamingCoordinator()
-                return EventSourceResponse(
-                    coordinator.stream_response_with_steward(request)
-                )
+                return EventSourceResponse(coordinator.stream_response_with_steward(request))
             else:
-                return EventSourceResponse(
-                    service.create_response_stream(request)
-                )
+                return EventSourceResponse(service.create_response_stream(request))
 
         # Non-streaming response
         if use_steward: