feat: implement G1 AI credits overage flow with billing telemetry

Adds end-to-end support for Google One AI credits in quota exhaustion flows:

- New billing module (packages/core/src/billing/) with credit balance
  checking, overage strategy management, and G1 URL construction
- OverageMenuDialog and EmptyWalletDialog UI components for quota
  exhaustion with credit purchase options
- Credits flow handler extracted to creditsFlowHandler.ts with overage
  menu, empty wallet, and auto-use-credits logic
- Server-side credit tracking: enabledCreditTypes on requests,
  consumed/remaining credits from streaming responses
- Billing telemetry events (overage menu shown, option selected, credits
  used, credit purchase click, API key updated)
- OpenTelemetry metrics for overage option and credit purchase counters
- Credit balance display in /stats command with refresh support
- Settings: general.overageStrategy (ask/always/never) for credit usage
- Error handling: INSUFFICIENT_G1_CREDITS_BALANCE as terminal error
  regardless of domain field presence
- Persistent info message after

packages/core/src/code_assist/codeAssist.test.ts

@@ -73,6 +73,8 @@ describe('codeAssist', () => {
         'session-123',
         'free-tier',
         'free-tier-name',
+        undefined,
+        mockConfig,
       );
       expect(generator).toBeInstanceOf(MockedCodeAssistServer);
     });
@@ -103,6 +105,8 @@ describe('codeAssist', () => {
         undefined, // No session ID
         'free-tier',
         'free-tier-name',
+        undefined,
+        mockConfig,
       );
       expect(generator).toBeInstanceOf(MockedCodeAssistServer);
     });