fix(agents): recognize flat JSON billing payloads and snake_case error codes (#74188)

* fix(agents): recognize flat JSON billing payloads and snake_case error codes

Two independent fixes for billing error detection:

1. isErrorPayloadObject/parseApiErrorInfo now recognize flat JSON like
   {"error":"string_code","message":"..."} where error is a string code
   at the top level, not just nested {"error":{"type":"...","message":"..."}}
   envelopes.

2. isBillingErrorMessage now matches "insufficient_balance" (underscore)
   and "Insufficient MBT balance" (one word between insufficient/balance)
   via two new patterns in the billing pattern list.

Together these prevent raw JSON from leaking to user-facing chat when
providers return 402-style flat payloads.

Fixes #74079

* fix(agents): remove redundant billing pattern and fix misleading regex comment

src/shared/assistant-error-format.ts

@@ -47,6 +47,10 @@ function isErrorPayloadObject(payload: unknown): payload is ErrorPayload {
         return true;
       }
     }
+    // Flat error payloads: {"error":"insufficient_balance","message":"..."}
+    if (typeof err === "string" && typeof record.message === "string") {
+      return true;
+    }
   }
   return false;
 }
@@ -165,6 +169,9 @@ export function parseApiErrorInfo(raw?: string): ApiErrorInfo | null {
     if (typeof err.message === "string") {
       errMessage = err.message;
     }
+  } else if (typeof payload.error === "string") {
+    // Flat error payloads: {"error":"insufficient_balance","message":"..."}
+    errType = payload.error;
   }
 
   return {