Use RESPX for HTTPX2 request routing

[adamtheturtle]

Summary

• register a collision-free RESPX backend for HTTPX2
• share Vuforia route matching, response delays, pass-through, and router lifecycle between HTTPX and HTTPX2
• preserve native HTTPX2 connection and timeout exceptions
• remove the bespoke HTTPX2 transports and client monkeypatching (269 net lines)

This applies the integration demonstrated in requests-mock-flask#1948 without requiring an upstream release. It simplifies the HTTPX2 support introduced in #3543.

Validation

• 23 focused HTTPX/HTTPX2 tests
• 1,951 non-real-backend tests (839 skipped)
• pre-commit, pre-push, and manual hook stages
• 100% branch coverage for the changed production modules
• HTTPX2 backend registration verified in both import orders with pytest-httpx2