Add GAM interceptor integration - frontend

[aram356]

Summary

• Adds client-side GAM interceptor that intercepts GPT slotRenderEnded events
• Renders Prebid creatives when GAM doesn't have matching line items configured
• Supports multiple rendering methods:
  • Iframe src replacement (unwrapped) - for iframe wrapper creatives
  • doc.write - for full HTML creatives
  • iframe.src - for adUrl-based creatives
  • pbjs.renderAd fallback
• Adds GamConfig interface for configuration via tsjs.setConfig({ gam: {...} })
• Forwards GAM config from core config to GAM integration via lazy loader
• Adds comprehensive tests for iframe attribute extraction and interceptor behavior

Configuration

tsjs.setConfig({
  gam: {
    enabled: true,
    bidders: ['mocktioneer'],  // Only intercept these bidders, empty = all
    forceRender: false         // Force render even if GAM has a line item
  }
});

Or via server-side config (injected as window.tsGamConfig).

Features

• Filtering by specific bidders
• Force rendering even when GAM has a line item
• Auto-initialization on module load
• Stats tracking for debugging (tsGam.getStats())

Test plan

• Verify npm test passes in crates/js/lib
• Verify npm run build succeeds
• Test GAM interceptor with a real GAM/Prebid setup:
  • Verify interceptor captures slotRenderEnded events
  • Verify Prebid creative renders when GAM returns empty slot
  • Verify bidder filtering works correctly
  • Verify forceRender option works

Dependencies

• Depends on Add GAM interceptor integration - backend #240 (GAM backend integration)

Closes #249
Related to #179