Consult nested mocks innermost-first on the httpx backend

[adamtheturtle]

Fixes #3544.

Problem

respx keeps every started router in a single process-wide list and consults them in registration order. Each MockVWS respx router ends with a catch-all which raises httpx.ConnectError (not AllMockedAssertionError), so it propagates out of Mocker.handler's loop and no later router is tried.

The result: with one MockVWS nested inside another, the outer mock answered and the inner mock was unreachable — a request to the URL the inner mock was created for was refused. The requests and httpx2 backends do the opposite, because their patches form a LIFO stack.

requests inner-url    401
requests outer-url    raised ConnectionError
httpx    inner-url    raised ConnectError     <-- the odd one out
httpx    outer-url    401
httpx2   inner-url    401
httpx2   outer-url    raised ConnectError

Fix

start_respx_router now moves each newly started router to the front of respx's router list, so nesting resolves innermost-first on every backend. Unregistering on stop already restores the outer router to the front, so the outer mock answers again once the inner one has stopped. Mocker.async_handler walks the same list, so async nesting is fixed too.

Test

TestVWS::test_nested_mocks in tests/mock_vws/test_respx_mock_usage.py, mirroring the existing httpx2 test of the same name.

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