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.formatassistanterrortext.test.ts

@@ -1,6 +1,6 @@
 import type { AssistantMessage } from "@mariozechner/pi-ai";
 import { describe, expect, it } from "vitest";
-import { formatAssistantErrorText } from "./pi-embedded-helpers.js";
+import { BILLING_ERROR_USER_MESSAGE, formatAssistantErrorText } from "./pi-embedded-helpers.js";
 
 describe("formatAssistantErrorText", () => {
   const makeAssistantError = (errorMessage: string): AssistantMessage =>
@@ -53,4 +53,19 @@ describe("formatAssistantErrorText", () => {
     );
     expect(formatAssistantErrorText(msg)).toBe("LLM error server_error: Something exploded");
   });
+  it("returns a friendly billing message for credit balance errors", () => {
+    const msg = makeAssistantError("Your credit balance is too low to access the Anthropic API.");
+    const result = formatAssistantErrorText(msg);
+    expect(result).toBe(BILLING_ERROR_USER_MESSAGE);
+  });
+  it("returns a friendly billing message for HTTP 402 errors", () => {
+    const msg = makeAssistantError("HTTP 402 Payment Required");
+    const result = formatAssistantErrorText(msg);
+    expect(result).toBe(BILLING_ERROR_USER_MESSAGE);
+  });
+  it("returns a friendly billing message for insufficient credits", () => {
+    const msg = makeAssistantError("insufficient credits");
+    const result = formatAssistantErrorText(msg);
+    expect(result).toBe(BILLING_ERROR_USER_MESSAGE);
+  });
 });