Add a first-class integration for classic httpx

[bagowix]

Motivation

The shipped transport integration targets httpx2, while a large part of the Python ecosystem—including the OpenAI and Anthropic SDKs—still uses classic httpx. The current LLM guide therefore has to use decorator-level protection and explicitly says the transport cannot be reused.

A native httpx transport would materially reduce adoption friction and make per-host protection transparent for existing clients.

Proposed scope

Add an optional interlock-cb[httpx] extra with sync and async transport wrappers built only on httpx's public transport API, mirroring the proven httpx2 behavior where appropriate.

Acceptance criteria

• Provide sync and async transport classes under interlock.integrations.httpx.
• Keep one breaker per request host and reject host-less URLs eagerly.
• Reuse the canonical status classifier defaults (429, 500, 502, 503, 504) with configurable statuses.
• Delegate close() / aclose() and preserve streaming response semantics.
• Add unit and loopback E2E tests for success, classified failures, transport exceptions, per-host isolation, open rejection before I/O, and cleanup.
• Test the minimum supported httpx version and latest version in CI.
• Add installation, API, integration, and LLM-SDK documentation.
• Keep the core dependency-free and avoid private httpx APIs.

Non-goals

• Replacing the existing httpx2 integration.
• Adding retry behavior inside the transport.