Add HTTP payment response hooks

[parvahuja]

Stacked on #178.

Summary

• add optional sync and async post-payment response hooks
• expose contextual credential creation, policy-checked raw sends, and one-shot refetch
• preserve streaming ownership, request metadata, cleanup, and shared lifecycle events

Testing

• uv run --all-extras --group dev ruff check .
• uv run --all-extras --group dev pyright
• uv run --all-extras --group dev pytest -q (812 passed, 41 skipped)

[parvahuja]

Superseded by #170, which consolidates the shared runtime, sync transport, instrumentation, response hooks, and receipt extensions into one draft.