Store route handlers directly

[adamtheturtle]

Route methods were recovered dynamically in each HTTP mocking backend, with the HTTPX2 path trusting the result through a Ty suppression.

This stores the handler callable in each route definition, binds it once to the API instance, and passes the resulting typed callback directly to Requests, HTTPX, and HTTPX2. There is no dynamic attribute lookup or equivalent indirection.

The behavior is covered through the existing public mock integration tests; there are no direct tests of the private routing implementation.

Validation:

• public Requests, RESPX, HTTPX2, and Flask usage tests: 179 passed, 2 skipped
• uv run prek run --all-files --hook-stage pre-push