feat: add scoped HTTPX instrumentation

[parvahuja]

Stack 7 of 8 · depends on #191 · next: #193

Summary

Add scoped global HTTPX instrumentation so Python harnesses can make existing HTTPX clients payment-aware without replacing their transports.

• context scope is local and restores cleanly
• process scope propagates to worker threads
• nested bindings and recursive payment-internal HTTP are suppressed safely
• ambiguous process runtimes fail closed
• thread primitives are patched only while process scope is active

Scope

HTTPX only. This does not instrument MCP globally.

Validation

• 929 passed, 41 skipped
• 36 combined adapter/instrumentation cases pass on every supported HTTPX version
• factory entry, standalone events, credential work, and teardown remain outside recursive instrumentation
• Ruff, formatting, Pyright, diff checks, and Python 3.11–3.14 CI pass

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