feat: add shared payment runtime

[parvahuja]

Stack 3 of 8 · depends on #188 · next: #189

Summary

• add a shared PaymentRuntime with one AnyIO-owned asyncio loop
• support borrowed methods plus sync, async, and async-context-manager factories
• expose sync/async lifecycle contexts, credential creation, challenge matching, and events
• defer callback-triggered close until active calls finish; external close cancels in-flight calls before method teardown

Lifecycle contract

• direct methods= are borrowed and must be loop-independent
• method_factories= initialize once and async context managers exit on the owned loop
• factories and methods must finish work they spawn before returning
• the runtime is single-use: once closed, it cannot be restarted

Why this shape

AnyIO owns the portal, loop thread, task submission, and loop cleanup. pympp only tracks runtime state, active root calls, and method teardown. The generic coroutine runner and detached-task lease model are intentionally private/out of scope.

This simplification removes 412 net lines from the previous draft. It also declares the tested AnyIO range (>=4.12,<5).

Scope

Runtime foundation only. Existing HTTP and MCP clients are unchanged; HTTPX instrumentation, origin policy, retry/outcome tracking, and MCP integration come in later PRs.

Validation

• full suite: 805 passed, 41 skipped on Python 3.13
• focused runtime and Tempo tests: 237 passed
• clean-wheel smoke: Python 3.11 with minimum AnyIO 4.12.0
• Ruff, formatting, Pyright, package build, strict Twine check, and dependency check pass

_{Managed as a stacked change with GitHub Stacks.}

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep async context managers in their entering task context

When a factory returns an async context manager that uses task-local state, this submits _teardown_methods() as a new task even though __aenter__ ran in the earlier _initialize_methods() task. For example, an @asynccontextmanager that sets a ContextVar and resets its token in finally consistently raises ValueError: Token was created in a different Context during close(), potentially interrupting cleanup. Keep the exit stack alive in one supervisor task that performs both entry and exit rather than resubmitting teardown separately.

AGENTS.md reference: AGENTS.md:L68-L69

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