#21920: fix(antigravity): map deprecated Gemini 3 Pro to Gemini 3.1 Pro

## Summary Fixes #21875 - Google deprecated Gemini 3 Pro in Google Antigravity. Users selecting a Gemini model via the `google-antigravity` provider get: *"Gemini 3 Pro is no longer available. Please switch to Gemini 3.1 Pro"* - The existing `normalizeGoogleModelId` function was never called for the `google-antigravity` provider path — only for the direct `google` (API key) provider - Adds `normalizeAntigravityModelId` that remaps `gemini-3-pro` and `gemini-3-pro-preview` to `gemini-3.1-pro` - Wires it into `normalizeProviderModelId` for the `google-antigravity` provider ## Test plan - [ ] CI passes - [ ] Configure `google-antigravity/gemini-3-pro` and verify it resolves to `gemini-3.1-pro` - [ ] Verify direct `google` provider (`gemini-3-pro-preview`) is unaffected