feat: add per-client HTTPX adapters

[parvahuja]

Stack 6 of 8 · depends on #190 · next: #192

Summary

• add wrap_client() and wrap_async_client() for existing HTTPX instances
• fail fast when the installed HTTPX seam is unsupported or the client kind is wrong
• make wrapping transactional: partial mutation is rolled back on failure
• declare and test support for HTTPX 0.27.x and 0.28.x

Scope

Explicit per-client opt-in only. Global instrumentation is intentionally deferred to #192.

Validation

• 910 passed, 41 skipped
• adapter suite passes on HTTPX 0.27.0, 0.27.1, 0.27.2, 0.28.0, and 0.28.1
• Ruff, formatting, Pyright, diff checks, and Python 3.11–3.14 CI pass

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