feat: share runtime with MCP clients

[parvahuja]

Stack 8 of 8 · depends on #192 · top of stack

Summary

Let explicit MCP clients share a PaymentRuntime, including origin policy, shared events, committed-retry close protection, repeated-challenge protection, and fail-closed handling for uncertain payment outcomes.

MCP session calls stay on the caller loop; credential and event work use the runtime-owned loop. Sync close is guarded inside a running caller event loop, while await client.aclose() is the safe async lifecycle path.

Scope

This is explicit MCP client integration only. It intentionally excludes global MCP instrumentation, speculative successful-response refetch hooks, and receipt extensions.

Validation

• 948 passed, 41 skipped
• 49 MCP tests pass against the minimum supported MCP SDK 1.19.0
• all 36 HTTPX adapter/instrumentation cases pass on HTTPX 0.27.0–0.28.1
• built-wheel public smoke covers sync/async HTTP, adapters, global instrumentation, MCP payment retry, loop ownership, and clean shutdown
• Ruff, formatting, Pyright, diff checks, and Python 3.11–3.14 CI pass

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