Add synchronous payment transport

[parvahuja]

Summary

• add SyncPaymentTransport and PaymentRuntime.wrap_client(...) for existing httpx.Client instances
• bridge async credential creation from sync callers through one lazy, closeable runtime loop
• preserve request bodies, client auth behavior, redirected challenge policy, and lazy paid response streams
• cover a real OpenAI sync streaming call from a bare worker thread

Stacked on #170.

Validation

• uv run --all-extras --group dev ruff check .
• uv run --all-extras --group dev ruff format --check .
• uv run --all-extras --group dev pyright
• uv run --all-extras --group dev pytest -q (805 passed, 41 skipped)

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	pypi/​openai@​2.46.0

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[parvahuja]

Folded into #170, which now contains the shared runtime, sync transport, instrumentation, response hooks, and receipt extensions as one draft.