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/telemetry/telemetryAttributes.ts

@@ -15,11 +15,13 @@ const installationManager = new InstallationManager();
 export function getCommonAttributes(config: Config): Attributes {
   const email = userAccountManager.getCachedGoogleAccount();
   const experiments = config.getExperiments();
+  const authType = config.getContentGeneratorConfig()?.authType;
   return {
     'session.id': config.getSessionId(),
     'installation.id': installationManager.getInstallationId(),
     interactive: config.isInteractive(),
     ...(email && { 'user.email': email }),
+    ...(authType && { auth_type: authType }),
     ...(experiments &&
       experiments.experimentIds.length > 0 && {
         'experiments.ids': experiments.experimentIds,