fix(errors): show clear billing error instead of cryptic API response (#8391)

* fix(errors): return clear billing error message instead of cryptic raw error (#8136)

When an LLM API provider returns a credit/billing-related error (HTTP 402,
insufficient credits, low balance, etc.), OpenClaw now shows a clear,
actionable message instead of passing through the raw/cryptic error text:

  ⚠️ API provider returned a billing error — your API key has run out of
  credits or has an insufficient balance. Check your provider's billing
  dashboard and top up or switch to a different API key.

Changes:
- formatAssistantErrorText: detect billing errors via isBillingErrorMessage()
  and return a user-friendly message (placed before the generic HTTP/JSON
  error fallthrough)
- sanitizeUserFacingText: same billing detection for the sanitization path
- pi-embedded-runner/run.ts: add billingFailure detection in the profile
  exhaustion fallback, so the FailoverError message is billing-specific
- Added 3 new tests for credit balance, HTTP 402, and insufficient credits

* fix: extract billing error message to shared constant

src/agents/pi-embedded-helpers.ts

@@ -6,6 +6,7 @@ export {
   stripThoughtSignatures,
 } from "./pi-embedded-helpers/bootstrap.js";
 export {
+  BILLING_ERROR_USER_MESSAGE,
   classifyFailoverReason,
   formatRawAssistantErrorForUi,
   formatAssistantErrorText,