fix(agent): handle upstream provider failures

[tatoalo]

Problem

Cloud task failures can surface raw upstream provider error payloads when the agent receives retryable provider failures. That makes task run errors harder to read and can leak noisy JSON into downstream notifications.

Changes

Classify retryable upstream provider HTTP failures from the SDK result messages when they include API Error: 429 or API Error: 5xx

[greptile-apps]

Prompt To Fix All With AI

Fix the following 3 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 3
packages/agent/src/server/question-relay.test.ts:98-105
**Missing `429` test case despite explicit regex support**

The PR description highlights handling `API Error: 429` as a key scenario, and `UPSTREAM_PROVIDER_ERROR_STATUS_PATTERN` explicitly includes `429` in the alternation — but the parameterised test never exercises it. If the regex alternation for `429` were accidentally dropped, this suite would not catch the regression.

### Issue 2 of 3
packages/agent/src/adapters/codex/codex-agent.test.ts:309-340
**Non-parameterised test; `agent_error` pass-through path not covered**

Per the project's simplicity rules, parameterised tests are always preferred. This test only exercises the 529 classification path through `classifyPromptError`. The `agent_error` branch (where the original error is returned unchanged) goes untested, and additional status codes like 429 or 503 are not covered. Converting to `it.each` would cover all meaningful inputs, including the pass-through branch, with no extra boilerplate.

### Issue 3 of 3
packages/agent/src/server/question-relay.test.ts:39-40
`UPSTREAM_PROVIDER_FAILURE_MESSAGE` is defined identically in `agent-server.ts` (line 72) and re-declared here, violating OnceAndOnlyOnce. Exporting it from `agent-server.ts` (or moving it to `error-classification.ts`) and importing it in the test keeps the two in sync automatically.

```suggestion
import { UPSTREAM_PROVIDER_FAILURE_MESSAGE } from "../server/agent-server";
```

_{Reviews (1): Last reviewed commit: "fix(agent): handle upstream provider fai..." | Re-trigger Greptile}