Added support for mounting sub-applications on APIRouter with proper
prefix handling:
1. Override mount() in APIRouter to automatically apply the router's
prefix to the mount path
2. Handle Mount routes in include_router() to apply the include prefix
This allows patterns like:
router = APIRouter(prefix='/api')
router.mount('/subapp', FastAPI())
app.include_router(router)
Which correctly routes requests to /api/subapp/...
Note: Mounts must be added before include_router() is called, as
include_router copies routes at call time. This is consistent with
how other routes work.
Fixes#10180
Jonathan Fulton