[vdeng-ai]

Fix three boundary cases that could report failed requests as successful or leave error responses waiting past their deadline.

• Reject HTTP 200 non-stream Responses with explicit errors or any supplied status other than completed, including cancelled, queued, in_progress, and unknown states. Preserve partial text, usage, and failure diagnostics; retain compatibility with providers that omit status.
• Recognize Chat Completions stream error payloads so a later [DONE] cannot turn them into success.
• Keep timeout and external cancellation active while reading stream HTTP error bodies; preserve timely HTTP errors and report stalled reads as 408.

Validation: npm run verify passes (typecheck, lint, 220 tests, production build). Added 29 regression cases, including status checks with text only, usage only, and both; malformed statuses; and explicit errors overriding completed.

Scope is limited to these three boundary fixes and their regression tests.