Flash Backend Definitions and Auto Mode Update

What Changed

Updated auto mode selection logic to respect backend definitions for Gemini 3.5 Flash and route to Flash GA model when useGemini3_5Flash flag is enabled. Modified model selection in ACP utilities and ModelDialog to pass useGemini3_5Flash parameter through configuration. Added conditional schema logic distinguishing preview vs GA access scenarios with different authentication type handling.

Operator Implication

Ensures Flash model routing respects backend definitions and authentication type. Differentiates behavior between USE_GEMINI auth (uses gemini-3.5-flash) and other auth methods (uses gemini-3-flash). Enables proper display name resolution in UI.

Receipt

• Commit title: 'Respect backend definitions for 3.5 flash and Update auto mode to use 3.5 flash when the flag is enabled. (#27645)' with 24 files changed (+463/-