feat: provider adapter conformance test suite

[arthurpanhku]

Why (roadmap: Next)

Every model provider is an egress destination that user prompts — often proprietary source — get routed to. Today src/providers/ (anthropic.ts, openaiCompatible.ts) has per-adapter tests, but there's no shared contract every provider must satisfy. That makes "should we trust a new provider?" a subjective call each time (cf. #109). A conformance suite turns it into an objective, reviewable gate.

Sketch

• A single parametrized suite in tests/providers/conformance.test.ts that runs the same assertions against each registered adapter.
• Contract covers: streaming token deltas, tool/function-call round-trips, error/timeout normalization, and — most importantly for governance — that every request goes through the egress chokepoint (src/providers/egress.ts + trustedBaseUrls.ts), honors redaction, and produces the expected audit events.
• A bypass-style assertion proving an adapter cannot reach a non-allow-listed base URL.
• Fixture/mock transport so no real network calls are made.

Acceptance

• Adding a new provider means implementing the adapter + passing the suite — no bespoke review of egress/redaction/audit behavior.
• Existing anthropic and openaiCompatible adapters pass unchanged (or gaps they expose are fixed).

Constraints

Contract is narrowing-only: it asserts governance invariants, it can't be satisfied by weakening a control. Design the assertion list in the PR description first.