fix(llm-gateway): improve type safety and datetime consistency

- Add type annotations for mypy compliance
- Use UTC-aware datetimes consistently (datetime.now(UTC))
- Add type: ignore comments for LiteLLM incomplete stubs
- Fix import ordering and formatting
- Update pyproject.toml mypy configuration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mcp-servers/llm-gateway/streaming.py

@@ -63,11 +63,13 @@ class StreamAccumulator:
     def start(self) -> None:
         """Mark stream start."""
         import time
+
         self._started_at = time.time()
 
     def finish(self) -> None:
         """Mark stream finish."""
         import time
+
         self._finished_at = time.time()
 
     def add_chunk(
@@ -193,7 +195,7 @@ def format_sse_chunk(chunk: StreamChunk) -> str:
     Returns:
         SSE-formatted string
     """
-    data = {
+    data: dict[str, Any] = {
         "id": chunk.id,
         "delta": chunk.delta,
     }
@@ -278,7 +280,9 @@ class StreamBuffer:
             yield chunk
 
 
-async def stream_to_string(stream: AsyncIterator[StreamChunk]) -> tuple[str, UsageStats | None]:
+async def stream_to_string(
+    stream: AsyncIterator[StreamChunk],
+) -> tuple[str, UsageStats | None]:
     """
     Consume a stream and return full content.
 
@@ -331,9 +335,7 @@ async def merge_streams(
         pending.add(task)
 
     while pending:
-        done, pending = await asyncio.wait(
-            pending, return_when=asyncio.FIRST_COMPLETED
-        )
+        done, pending = await asyncio.wait(pending, return_when=asyncio.FIRST_COMPLETED)
 
         for task in done:
             idx, chunk = task.result()