* Move OpenAPI to providers/openapi/ submodule
Migrates the OpenAPI implementation to the provider pattern:
- Creates src/fastmcp/server/providers/openapi/ submodule with provider.py,
components.py, and routing.py
- Converts server/openapi/ to deprecated re-export stubs
- FastMCPOpenAPI remains as deprecated wrapper that uses OpenAPIProvider
- Updates experimental/server/openapi to import from new canonical location
* Update tests and FastMCP methods to use OpenAPIProvider
- Update FastMCP.from_openapi() and from_fastapi() to use OpenAPIProvider
directly, returning FastMCP instead of deprecated FastMCPOpenAPI
- Move tests from tests/server/openapi/ to tests/server/providers/openapi/
- Update all test imports to use new canonical location
- Add tests/deprecated/openapi/ for backward compatibility testing
- Update tests/client/test_openapi.py to use new imports
* Fix deprecation warning tests and address CodeRabbit feedback
- Use importlib.reload() to ensure deprecation warnings fire in tests
- Change logger.error to logger.exception for better stack traces
