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/models.py

@@ -6,7 +6,7 @@ Per ADR-004: LLM Provider Abstraction.
 """
 
 from dataclasses import dataclass, field
-from datetime import datetime
+from datetime import UTC, datetime
 from enum import Enum
 from typing import Any
 
@@ -282,7 +282,9 @@ class CompletionRequest(BaseModel):
     model_override: str | None = Field(
         default=None, description="Specific model to use (bypasses routing)"
     )
-    max_tokens: int = Field(default=4096, ge=1, le=32768, description="Max output tokens")
+    max_tokens: int = Field(
+        default=4096, ge=1, le=32768, description="Max output tokens"
+    )
     temperature: float = Field(
         default=0.7, ge=0.0, le=2.0, description="Sampling temperature"
     )
@@ -330,7 +332,7 @@ class CompletionResponse(BaseModel):
     )
     usage: UsageStats = Field(default_factory=UsageStats, description="Token usage")
     created_at: datetime = Field(
-        default_factory=datetime.utcnow, description="Response timestamp"
+        default_factory=lambda: datetime.now(UTC), description="Response timestamp"
     )
     metadata: dict[str, Any] = Field(
         default_factory=dict, description="Additional metadata"
@@ -354,9 +356,7 @@ class EmbeddingRequest(BaseModel):
     project_id: str = Field(..., description="Project ID for cost attribution")
     agent_id: str = Field(..., description="Agent ID making the request")
     texts: list[str] = Field(..., min_length=1, description="Texts to embed")
-    model: str = Field(
-        default="text-embedding-3-large", description="Embedding model"
-    )
+    model: str = Field(default="text-embedding-3-large", description="Embedding model")
 
 
 class EmbeddingResponse(BaseModel):
@@ -377,7 +377,7 @@ class CostRecord:
     prompt_tokens: int
     completion_tokens: int
     cost_usd: float
-    timestamp: datetime = field(default_factory=datetime.utcnow)
+    timestamp: datetime = field(default_factory=lambda: datetime.now(UTC))
     session_id: str | None = None
     request_id: str | None = None