test(session/retry): rename APIError test to 5xx to match 503 status

Copilot review feedback — test name said '500' but constructed error uses
statusCode 503.

Addresses audit finding F5 (Opus diamond review, 2026-04-22).

Author: tesdal

packages/opencode/test/session/retry-classification.test.ts

@@ -14,7 +14,7 @@ describe("SessionRetry.classify", () => {
     expect(out).toEqual({ message: "SSE read timed out after 120000ms", isTransport: true })
   })
 
-  test("returns non-transport classification for APIError 500", () => {
+  test("returns non-transport classification for APIError 5xx", () => {
     const err = new MessageV2.APIError({
       message: "upstream exploded",
       statusCode: 503,