[fastapi] Add router-level middleware support

[lry3069-afk]

Summary

Implement router-level middleware support for FastAPI's APIRouter. This allows middleware to be scoped to specific routers rather than applying globally to the entire application.

Changes

fastapi/routing.py

• Added middleware parameter to APIRouter.__init__ (after strict_content_type)
• Added self.middleware: List[Middleware] = [] initialization before super().__init__()
• Added add_middleware() instance method for runtime middleware registration
• Added middleware mount logic in include_router(): when middleware is present, routes are wrapped in a Mount path with the middleware applied, preventing direct route matches from bypassing middleware
• Conditional route registration: routes are added directly to self.routes only when no middleware is present; otherwise they go through the middleware Mount

fastapi/applications.py

• Added middleware parameter to FastAPI.include_router() signature
• Passed middleware=middleware to self.router.include_router() call

tests/test_router_middleware.py

• 7 test cases covering:
  • Router init with middleware parameter
  • Router init with no middleware (default)
  • Runtime middleware registration via add_middleware()
  • Middleware applied after routes are added
  • Middleware passed via include_router() parameter
  • Middleware scoped to child router routes
  • Multiple middleware execution order (LIFO)

Testing

python -X utf8 -m pytest fastapi/tests/test_router_middleware.py fastapi/tests/test_application.py -v
# Result: 15 passed in 0.67s

close #796

[lry3069-afk]

/claim #796

[lry3069-afk]

/claim #835

[github-actions]

Unfortunately the changes in this PR didn't fully resolve the issue. Please rework your solution and submit a new pull request.

Make sure to review the acceptance criteria in the linked issue and verify all conditions are met before resubmitting. See CONTRIBUTING.md for guidelines.