fix: trigger model failover for generic provider error messages

When Anthropic returns a generic error like 'An unknown error occurred'
without a type field, classifyFailoverReason() now recognizes it as a
transient failure and returns 'timeout' to trigger model failover.

Also handles other generic error patterns: 'an error occurred',
'internal server error', and 'service unavailable'.

Fixes #49706

src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts

@@ -860,4 +860,17 @@ describe("classifyFailoverReason", () => {
       ),
     ).toBe("timeout");
   });
+  it("classifies generic provider errors as timeout", () => {
+    expect(classifyFailoverReason("An unknown error occurred")).toBe("timeout");
+    expect(classifyFailoverReason("An error occurred")).toBe("timeout");
+    expect(classifyFailoverReason("Internal server error")).toBe("timeout");
+    expect(classifyFailoverReason("Service unavailable")).toBe("timeout");
+    // Case-insensitive
+    expect(classifyFailoverReason("AN UNKNOWN ERROR OCCURRED")).toBe("timeout");
+    expect(classifyFailoverReason("an error occurred while processing")).toBe("timeout");
+    // Wrapped in provider payload
+    expect(classifyFailoverReason('{"error":{"message":"An unknown error occurred"}}')).toBe(
+      "timeout",
+    );
+  });
 });

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

@@ -846,6 +846,19 @@ export function isBillingAssistantError(msg: AssistantMessage | undefined): bool
   return isBillingErrorMessage(msg.errorMessage ?? "");
 }
 
+function isGenericProviderError(raw: string): boolean {
+  if (!raw) {
+    return false;
+  }
+  const lower = raw.toLowerCase();
+  return (
+    lower.includes("an unknown error occurred") ||
+    lower.includes("an error occurred") ||
+    lower.includes("internal server error") ||
+    lower.includes("service unavailable")
+  );
+}
+
 function isJsonApiInternalServerError(raw: string): boolean {
   if (!raw) {
     return false;
@@ -1024,6 +1037,9 @@ export function classifyFailoverReason(raw: string): FailoverReason | null {
   if (isAuthErrorMessage(raw)) {
     return "auth";
   }
+  if (isGenericProviderError(raw)) {
+    return "timeout";
+  }
   return null;
 }