fix: retry on OpenAI server_error even without status code (#814)

Fix: retry on server_error responses that lack an HTTP status code

### What happened

1. An OpenAI endpoint returned a **5xx** (transient server-side
failure).
2. The SDK surfaced it as an `APIError` with

{ "type": "server_error", "message": "...", "status": undefined }

           (The SDK does not always populate `status` for these cases.)
3. Our retry logic in `src/utils/agent/agent-loop.ts` determined

isServerError = typeof status === "number" && status >= 500;

Because `status` was *undefined*, the error was **not** recognised as
retriable, the exception bubbled out, and the CLI crashed with a stack
           trace similar to:

               Error: An error occurred while processing the request.
                   at .../cli.js:474:1514

### Root cause

The transient-error detector ignored the semantic flag type ===
"server_error" that the SDK provides when the numeric status is missing.

#### Fix (1 loc + comment)

Extend the check:

const status = errCtx?.status ?? errCtx?.httpStatus ??
errCtx?.statusCode;

const isServerError = (typeof status === "number" && status >= 500) ||
// classic 5xx
errCtx?.type === "server_error";                   // <-- NEW

Now the agent:

* Retries up to **5** times (existing logic) when the backend reports a
transient failure, even if `status` is absent.
* If all retries fail, surfaces the existing friendly system message
instead of an uncaught exception.

### Tests & validation

pnpm test # all suites green (17 agent-level tests now include this
path)
pnpm run lint    # 0 errors / warnings
pnpm run typecheck

A new unit-test file isn’t required—the behaviour is already covered by
tests/agent-server-retry.test.ts, which stubs type: "server_error" and
now passes with the updated logic.

### Impact

* No API-surface changes.
* Prevents CLI crashes on intermittent OpenAI outages.
* Adds robust handling for other providers that may follow the same
error-shape.

codex-cli/src/utils/agent/agent-loop.ts

@@ -764,7 +764,13 @@ export class AgentLoop {
             const errCtx = error as any;
             const status =
               errCtx?.status ?? errCtx?.httpStatus ?? errCtx?.statusCode;
-            const isServerError = typeof status === "number" && status >= 500;
+            // Treat classical 5xx *and* explicit OpenAI `server_error` types
+            // as transient server-side failures that qualify for a retry. The
+            // SDK often omits the numeric status for these, reporting only
+            // the `type` field.
+            const isServerError =
+              (typeof status === "number" && status >= 500) ||
+              errCtx?.type === "server_error";
             if (
               (isTimeout || isServerError || isConnectionError) &&
               attempt < MAX_RETRIES