Add Chat Completions wrapper with reasoning conversion

Implements Phase 5: OpenAI Chat Completions compatibility layer

Features:
- Wraps Responses API for single source of truth
- Automatically enables reasoning generation
- Converts reasoning items to <think> tags for Open WebUI
- Maintains OpenAI-compatible chat completion format
- Supports both streaming and non-streaming modes
- Pipeline prefix preservation for model names
- System message handling

Architecture:
- Service layer calls Responses API internally
- Streams word-by-word for smooth UX
- Reasoning displayed in thought bubbles (Open WebUI)
- Main response shown separately from thinking

Error Handling:
- Enhanced exception types (RateLimitError, ContextLengthError)
- OpenAI-compatible error format
- Graceful error propagation from Responses API

Testing:
- 6 unit tests for chat router functionality
- 6 unit tests for streaming wrapper behavior
- Total: 12 tests with comprehensive coverage

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

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

src/core/exceptions.py

@@ -47,6 +47,27 @@ class ModelNotFoundError(AppException):
 
 class ValidationError(AppException):
     """Raised for validation errors."""
-    
+
     def __init__(self, message: str, details: dict[str, Any] | None = None):
         super().__init__(message=message, status_code=422, details=details)
+
+
+class RateLimitError(AppException):
+    """Raised when rate limit is exceeded."""
+
+    def __init__(self, message: str = "Rate limit exceeded"):
+        super().__init__(message=message, status_code=429)
+
+
+class ContextLengthError(AppException):
+    """Raised when context length exceeds model limits."""
+
+    def __init__(self, message: str = "Context length exceeded"):
+        super().__init__(message=message, status_code=400)
+
+
+class APIError(AppException):
+    """Generic API error."""
+
+    def __init__(self, message: str, status_code: int = 500):
+        super().__init__(message=message, status_code=status_code)