fix(provider): extract nested error.message from OpenAI-shaped bodies

[chandlerm923]

Issue for this PR

Closes #36410

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

message() in packages/opencode/src/provider/error.ts extracts a fallback error string from the response body with:

const errMsg = body.message || body.error || body.error?.message

For the common OpenAI-shaped body {"error":{"message":"..."}}, body.error is a truthy object, so it wins the || chain before body.error.message is ever read. The typeof errMsg === "string" check then fails and the real message is dropped — users see a generic status line or a raw JSON dump instead of the actual reason (rate limit, quota, invalid key, etc.).

There was an earlier PR for this (#36411) that implemented essentially the same fix, but it went stale and was closed by the automated cleanup bot (>1 month old, no reactions) rather than rejected on the merits — no maintainer raised any objection to the approach there.

This PR reorders the extraction to type-check each field explicitly and prefer the nested string first:

const errMsg =
  (typeof body.error?.message === "string" ? body.error.message : undefined) ??
  (typeof body.message === "string" ? body.message : undefined) ??
  (typeof body.error === "string" ? body.error : undefined)

How did you verify your code works?

• Added two tests to packages/opencode/test/provider/error.test.ts: one for the nested {error:{message}} shape, one for the pre-existing top-level string error shape (to make sure that path still works).
• bun test test/provider/error.test.ts — both new tests pass.
• bun turbo typecheck — 30/30 packages pass.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[Enough1122]

AI code review — automated review for reference; please use your judgment.

1. packages/opencode/src/provider/error.ts:52 — An empty-string nested message breaks the ?? chain: { "error": { "message": "" }, "message": "real reason" } makes the first operand "" (not nullish), so coalescing never reaches body.message, and the outer if (errMsg) then discards it — net result: no extraction at all, whereas the old code returned "real reason". Why it matters: some gateways emit empty nested messages with a usable top-level one. Suggestion: coerce empties to undefined before coalescing, e.g. (s) => (typeof s === "string" && s.trim() ? s : undefined) applied per candidate.

2. packages/opencode/src/provider/error.ts:51 — Precedence flip worth noting: previously body.message won over body.error; now nested error.message always wins. That's right for OpenAI-shaped bodies, but providers returning both fields with different texts will surface a different message than before. Why it matters: users/tests relying on the old text see a change. Suggestion: fine to keep — just call it out in the PR body/changelog.

3. packages/opencode/test/provider/error.test.ts:6 — Missing cases: (a) both message and error.message present (locks in the new precedence), (b) { message } only, (c) the empty-string nested message from item 1 once fixed. Why it matters: these are exactly the branches the reorder touched. Suggestion: add the three table-driven cases.

4. packages/opencode/src/provider/error.ts:55 (nit) — Common alternate shapes (detail from FastAPI-style APIs, errors[0].message) remain unhandled. Not blocking; refactoring to an ordered candidates list would make future additions one-liners.

— automated review (ox-alpha, round2)