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-runner/run.ts

@@ -29,9 +29,11 @@ import {
 import { normalizeProviderId } from "../model-selection.js";
 import { ensureOpenClawModelsJson } from "../models-config.js";
 import {
+  BILLING_ERROR_USER_MESSAGE,
   classifyFailoverReason,
   formatAssistantErrorText,
   isAuthAssistantError,
+  isBillingAssistantError,
   isCompactionFailureError,
   isContextOverflowError,
   isFailoverAssistantError,
@@ -549,6 +551,7 @@ export async function runEmbeddedPiAgent(
 
           const authFailure = isAuthAssistantError(lastAssistant);
           const rateLimitFailure = isRateLimitAssistantError(lastAssistant);
+          const billingFailure = isBillingAssistantError(lastAssistant);
           const failoverFailure = isFailoverAssistantError(lastAssistant);
           const assistantFailoverReason = classifyFailoverReason(lastAssistant?.errorMessage ?? "");
           const cloudCodeAssistFormatError = attempt.cloudCodeAssistFormatError;
@@ -620,9 +623,11 @@ export async function runEmbeddedPiAgent(
                   ? "LLM request timed out."
                   : rateLimitFailure
                     ? "LLM request rate limited."
-                    : authFailure
-                      ? "LLM request unauthorized."
-                      : "LLM request failed.");
+                    : billingFailure
+                      ? BILLING_ERROR_USER_MESSAGE
+                      : authFailure
+                        ? "LLM request unauthorized."
+                        : "LLM request failed.");
               const status =
                 resolveFailoverStatus(assistantFailoverReason ?? "unknown") ??
                 (isTimeoutErrorMessage(message) ? 408 : undefined);