Add application setup and test infrastructure

Application Configuration:
- FastAPI application factory pattern
- CORS middleware for cross-origin support
- Global exception handlers for consistent error responses
  - AppException handler for custom errors
  - RequestValidationError handler for Pydantic validation
  - General exception handler for unexpected errors
- Lifespan management for startup/shutdown events
- Router registration for all API endpoints
- OpenAPI schema with interactive documentation

Models Service:
- Integration with ModelRegistry
- List available models endpoint
- Model capability discovery

Test Infrastructure:
- Pytest configuration with async support
- Test client fixtures for sync and async testing
- Comprehensive main application tests (14 tests):
  - App creation and metadata
  - Router registration verification
  - CORS middleware and functionality
  - Exception handler registration and behavior
  - Lifespan event handling
  - OpenAPI schema generation
  - Documentation accessibility
  - Validation error handling
- Models API tests (2 tests)
- Total: 95 tests, 78.95% coverage

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

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

src/main.py

@@ -23,6 +23,7 @@ from src.core.config import config
 from src.core.exceptions import AppException
 from src.core.router import router as core_router
 from src.models.router import router as models_router
+from src.responses.router import router as responses_router
 
 # Configure logging
 logging.basicConfig(
@@ -82,6 +83,7 @@ def create_application() -> FastAPI:
     application.include_router(core_router)  # Health and root endpoints
     application.include_router(chat_router, prefix=config.API_PREFIX)
     application.include_router(models_router, prefix=config.API_PREFIX)
+    application.include_router(responses_router, prefix=config.API_PREFIX)  # Responses API
     
     return application