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/tests/test_streaming.py

@@ -110,6 +110,7 @@ class TestWrapLiteLLMStream:
 
     async def test_wrap_stream_basic(self) -> None:
         """Test wrapping a basic stream."""
+
         # Create mock stream chunks
         async def mock_stream():
             chunk1 = MagicMock()
@@ -146,6 +147,7 @@ class TestWrapLiteLLMStream:
 
     async def test_wrap_stream_without_accumulator(self) -> None:
         """Test wrapping stream without accumulator."""
+
         async def mock_stream():
             chunk = MagicMock()
             chunk.choices = [MagicMock()]
@@ -284,6 +286,7 @@ class TestStreamToString:
 
     async def test_stream_to_string_basic(self) -> None:
         """Test converting stream to string."""
+
         async def mock_stream():
             yield StreamChunk(id="1", delta="Hello")
             yield StreamChunk(id="2", delta=" ")
@@ -303,6 +306,7 @@ class TestStreamToString:
 
     async def test_stream_to_string_no_usage(self) -> None:
         """Test stream without usage stats."""
+
         async def mock_stream():
             yield StreamChunk(id="1", delta="Test")