Inherit Starlette router dispatch - #16236

Verified locally on the PR branch (Python 3.13, editable install of the branch):

Behavioral parity probe (branch vs release 0.141.1) — four contracts, identical results:

Targeted test files: test_router_redirect_slashes.py, test_router_prefix_with_template.py, test_empty_router.py, test_router_events.py, test_include_router_defaults_overrides.py, test_ws_router.py — 70/70 green.

Checked the wiring details:

1. Starlette's Router.app sets scope["router"] = self just like the removed fastapi copy did — no loss for middleware/introspection that reads it.
2. Match ordering is preserved: starlette's loop handles FULL/PARTIAL + its own redirect_slashes pass, then calls self.default — which now points at _handle_no_match, so fastapi's low-priority route logic runs at the same point in the sequence as before, and _default keeps the original 404 handler.
3. Deleting ~40 lines of drifted copy in favor of upstream's dispatch means future starlette routing fixes flow through automatically instead of needing manual sync — clearly the right direction.

One non-blocking nit: self._default = self.default; self.default = self._handle_no_match captures the original default at __init__ time, so a subclass that overrides .default after super().__init__() would have its override bypassed (_handle_no_match calls the captured _default). Extremely unlikely to matter in practice, but worth a comment line if you want to guard it.

CI fully green. Approving.