Add GAM interceptor integration - frontend

[aram356]

Description

Add client-side GAM interceptor that intercepts GPT slotRenderEnded events to render Prebid creatives.

Changes

• Add GAM interceptor module (integrations/gam/index.ts)
• Support multiple rendering methods: iframe src replacement, doc.write, pbjs.renderAd
• Add GamConfig interface for configuration via tsjs.setConfig({ gam: {...} })
• Forward GAM config from core config to GAM integration via lazy loader
• Add comprehensive tests

Features

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

Why

Enables rendering Prebid creatives when GAM doesn't have matching line items, without requiring GAM trafficking.

Related

• PR: Add GAM interceptor integration - frontend #241
• Depends on: Add GAM interceptor integration - backend #248
• Parent: Unable to render prebidjs ads with NextJS #179