fix(auth): don't cool down an account for a request-scoped 4xx

Do not trigger account cooldown or fallback for request-scoped 4xx errors that match no account rules so healthy credentials are not locked out for context length or validation errors.

tests/unit/account-fallback-4xx.test.js

@@ -0,0 +1,38 @@
+// Regression: an unmatched 4xx (a request-scoped failure) used to hit the
+// transient-cooldown default, which locked the account for 30s and — with a
+// single connection — answered every other request in that window with a copy of
+// the first error. A 400 "maximum context length" from one session therefore
+// looked like the same failure in unrelated sessions.
+import { describe, expect, it } from "vitest";
+import { checkFallbackError } from "../../open-sse/services/accountFallback.js";
+
+describe("checkFallbackError — request-scoped vs account-scoped failures", () => {
+  it("does not cool the account down for a 400 caused by the request", () => {
+    const result = checkFallbackError(400, JSON.stringify({
+      error: {
+        message: "This model's maximum context length is 1048576 tokens. However, you requested 1186139 tokens",
+        type: "invalid_request_error",
+      },
+    }));
+
+    expect(result).toEqual({ shouldFallback: false, cooldownMs: 0 });
+  });
+
+  it("still falls back for account-scoped statuses", () => {
+    for (const status of [401, 402, 403, 404, 429]) {
+      expect(checkFallbackError(status, "nope").shouldFallback).toBe(true);
+    }
+  });
+
+  it("still honours rate-limit / quota wording on any 4xx", () => {
+    expect(checkFallbackError(400, "rate limit reached").shouldFallback).toBe(true);
+    expect(checkFallbackError(422, "quota exceeded").shouldFallback).toBe(true);
+  });
+
+  it("keeps the transient cooldown for unmatched server errors", () => {
+    const result = checkFallbackError(503, "upstream exploded");
+
+    expect(result.shouldFallback).toBe(true);
+    expect(result.cooldownMs).toBeGreaterThan(0);
+  });
+});