refactor(agents): share failover HTTP status classification (#36615)

* fix(agents): classify transient failover statuses consistently

* fix(agents): preserve legacy failover status mapping

src/agents/pi-embedded-helpers/errors.ts

@@ -251,6 +251,43 @@ export function isTransientHttpError(raw: string): boolean {
   return TRANSIENT_HTTP_ERROR_CODES.has(status.code);
 }
 
+export function classifyFailoverReasonFromHttpStatus(
+  status: number | undefined,
+  message?: string,
+): FailoverReason | null {
+  if (typeof status !== "number" || !Number.isFinite(status)) {
+    return null;
+  }
+
+  if (status === 402) {
+    return "billing";
+  }
+  if (status === 429) {
+    return "rate_limit";
+  }
+  if (status === 401 || status === 403) {
+    if (message && isAuthPermanentErrorMessage(message)) {
+      return "auth_permanent";
+    }
+    return "auth";
+  }
+  if (status === 408) {
+    return "timeout";
+  }
+  // Keep the status-only path conservative and behavior-preserving.
+  // Message-path HTTP heuristics are broader and should not leak in here.
+  if (status === 502 || status === 503 || status === 504) {
+    return "timeout";
+  }
+  if (status === 529) {
+    return "rate_limit";
+  }
+  if (status === 400) {
+    return "format";
+  }
+  return null;
+}
+
 function stripFinalTagsFromText(text: string): string {
   if (!text) {
     return text;