Replay Harness LLM Gateway Fake-Upstream Oracle

LIFECYCLE: permanent — a deterministic, advisory structural test for the LLM gateway's real route and provider terminal path. Run it with:

npm run test:replay-llm-fake-upstream

The oracle starts a loopback-only OpenAI-compatible fake upstream and injects that upstream only through FastAPI's test dependency override. It then drives the real gateway router, route resolver, executor, and OpenAICompatibleChatCompletionProvider. Normal provider construction and routing remain unchanged outside this test process.

Contract

The fake upstream proves only that:

1. the real provider adapter uses the expected chat-completions path;
2. a routed gateway request completes exactly one proxy-isolated, bounded local round trip;
3. gateway and upstream event ordering remains stable; and
4. an incoming request cannot choose or redirect the upstream target.

It intentionally does not establish OpenAI or Anthropic SSE fidelity, provider conformance, production endpoint behavior, client compatibility, a release gate, capture/replay, or Phase 0B.

Data safety

The fake retains only endpoint path shape, event labels, request/response schema keys, status/error classes, request count, and a bounded-timing result. It never logs or persists prompt content, completions, token values, provider bodies, header values, credentials, or identifiers.