* 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
ef7c528c8a