feat: add per-client HTTPX adapters

[parvahuja]

Runtime stack 4 of 5 · depends on #190 · next: #192

Summary

• add wrap_client() and wrap_async_client() for existing HTTPX instances
• preserve configured transports, auth, redirects, cookies, hooks, streaming, and send callables cached before adaptation
• enforce one payment across each complete HTTPX auth/redirect flow
• fail before mutation on unsupported HTTPX versions or adapter seams
• roll back partial client mutation transactionally

Review boundary

Explicit per-client opt-in only. Each client keeps one payment transport, and one nested-safe context variable spans HTTPX auth and redirect flows. The adapter patches _send_handling_auth and _send_single_request; it does not patch the public send method. Wrap clients before concurrent use.

Compatibility

Supported and tested: HTTPX 0.27.x and 0.28.x.

Validation

• full suite: 925 passed, 41 skipped
• adapter suite: 26 passed on HTTPX 0.27.0 and 0.28.1
• cached bound sends, custom multi-request auth, redirects, nested hooks, request reuse, cancellation, and uncertain-outcome smokes
• Ruff, formatting, and Pyright pass