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/errors.ts

@@ -3,6 +3,9 @@ import type { OpenClawConfig } from "../../config/config.js";
 import type { FailoverReason } from "./types.js";
 import { formatSandboxToolPolicyBlockedMessage } from "../sandbox.js";
 
+export const BILLING_ERROR_USER_MESSAGE =
+  "⚠️ 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.";
+
 export function isContextOverflowError(errorMessage?: string): boolean {
   if (!errorMessage) {
     return false;
@@ -368,6 +371,10 @@ export function formatAssistantErrorText(
     return "The AI service is temporarily overloaded. Please try again in a moment.";
   }
 
+  if (isBillingErrorMessage(raw)) {
+    return BILLING_ERROR_USER_MESSAGE;
+  }
+
   if (isLikelyHttpErrorText(raw) || isRawApiErrorPayload(raw)) {
     return formatRawAssistantErrorForUi(raw);
   }
@@ -403,6 +410,10 @@ export function sanitizeUserFacingText(text: string): string {
     );
   }
 
+  if (isBillingErrorMessage(trimmed)) {
+    return BILLING_ERROR_USER_MESSAGE;
+  }
+
   if (isRawApiErrorPayload(trimmed) || isLikelyHttpErrorText(trimmed)) {
     return formatRawAssistantErrorForUi(trimmed);
   }