fix(error-classifier): add insufficient balance to billing patterns

DeepSeek API returns HTTP 400 with 'Insufficient Balance' message when
account funds are depleted. This pattern was not in _BILLING_PATTERNS,
causing the error to be misclassified instead of triggering billing
exhaustion handling (e.g., fallback to alternate provider).

Suggested by teknium1 in PR review of #15586.

agent/error_classifier.py

@@ -91,6 +91,7 @@ class ClassifiedError:
 _BILLING_PATTERNS = [
     "insufficient credits",
     "insufficient_quota",
+    "insufficient balance",
     "credit balance",
     "credits have been exhausted",
     "top up your credits",