fix: preserve request extensions on x402 retry

[shuofengzhang]

What changed

• Preserve httpx.Request.extensions when cloning a request for x402 retry headers in _clone_request_with_headers.
• Add transport unit tests to verify:
  • retry header merging/overrides behave correctly
  • request extensions are preserved in the cloned retry request

Why

• httpx stores per-request metadata (for example timeout settings) in Request.extensions.
• The retry clone previously rebuilt a request without copying extensions, which could drop request-level metadata on the paid retry.
• Preserving extensions keeps retry behavior consistent with the original request while remaining fully backward-compatible.

Testing

• ✅ scripts/clone_and_test.sh qntx/x402-openai-python (pass)
• ✅ source .venv/bin/activate && pytest -q (25 passed)