Backend-aware advice in HTTP errors: key variable, 402, Retry-After

[DevMortimer]

What and why

HTTP error advice always said Check TYPESAFE_API_KEY., even when the client talked to OpenRouter, where the key comes from OPENROUTER_API_KEY. A 402 fell through to the generic "try again later" text, and a 429 ignored the server's Retry-After header. The advice now matches the backend that was called.

Change

• safeError(error, backend?) takes an optional backend name or config. The client passes its backend from evaluate() and listModels().
• 401: Check <keyEnv>. for the backend's key variable (OPENROUTER_API_KEY on OpenRouter). Without a backend, the text stays Check TYPESAFE_API_KEY..
• 402: Insufficient credits. Add credits at https://openrouter.ai/credits. on OpenRouter, Check your account balance. elsewhere. A 402 does not mark the key unusable.
• 429: appends Retry after <n> seconds. when the response carries a numeric Retry-After. Dates, blanks, and other values are ignored, so the message quotes no other header content.
• docs/api.md and CHANGELOG.md updated.

Verification

• npm ci and npm run check pass: build, typecheck, 102 offline tests (2 new). The base branch passes its 100.
• The new tests run offline through the injected fetch on both backends: 401, 402, and 429 with and without Retry-After: 7. They assert the exact messages, that usable is false after a 401 and true after a 402, and that no upstream body reaches a message.
• Documentation and changelog notes updated.

Live testing is not applicable: the transport and response validation are unchanged; only the text of error messages changes.

Compatibility and release notes

• Additive: the new backend parameter is optional, and one-argument safeError calls return the same messages as before for 401 and 429 without Retry-After.
• The 402 message changes from the generic retry advice to billing advice.
• Limitation: OpenRouter is detected by its configured host, so a custom backend on another OpenRouter-compatible host gets the generic 402 text.
• No version bump.