fix: harden async payment retries

[parvahuja]

Summary

• Parse merged WWW-Authenticate fields correctly, including multiple schemes and quoted commas.
• Pass one-shot async bodies through for ordinary responses and reject them only after a payable 402 requires replay.
• Close failed challenge reads and surface cancelled or failed credentialed retries as the shared PaymentOutcomeUnknownError.
• Reuse that same outcome error from the existing MCP client instead of maintaining a second type.

This remains async and transport-local. It adds no sync bridge, global instrumentation, thread lifecycle, or origin policy.

Stack: 2 of 2, on #201. The repository's full CI intentionally runs only for PRs targeting main; it will run here after #201 merges and this PR is retargeted.

Validation

• 797 passed, 41 skipped
• Ruff check and format check
• Pyright
• Clean diff check
• Hermes Agent 0.19.0 plugin-loader smoke on HTTPX 0.27.2 and 0.28.1: sync, async, pre-existing clients, worker threads, origin policy, recursion bypass, repeated/concurrent challenge protection, uncertain outcomes, and in-flight shutdown all passed