Gemini 3.5 Flash GA routes to flagged users via backend experiment flag, no client update

What this changes for operators

• Operators auditing which model their CLI calls cannot rely on client version alone - model selection is now gated server-side by experiment flag GEMINI_3_5_FLASH_GA_LAUNCHED (ID 45780819) via hasGemini35FlashGAAccess().
• Auto-routing logic silently switches to Flash GA when the flag is enabled for a user cohort, so the same binary can route to different models across users.
• Verification path: Config.hasGemini35FlashGAAccess() and the registered experiment flag determine routing; the model in use is no longer fully determined by local config.
• Single decision: operators must treat backend flag state as part of the model-routing audit surface.