* fix(provider): normalize bare gemini-3 Pro model IDs for google-antigravity
The Antigravity Cloud Code Assist API requires a thinking-tier suffix
(-low or -high) for all Gemini 3 Pro variants. When a user configures
a bare model ID like `gemini-3.1-pro`, the API returns a 404 because it
only recognises `gemini-3.1-pro-low` or `gemini-3.1-pro-high`.
Add `normalizeAntigravityModelId()` that appends `-low` (the default
tier) to bare Pro model IDs, and apply it during provider normalisation
for `google-antigravity`. Also refactor the per-provider model
normalisation into a shared `normalizeProviderModels()` helper.
Closes#24071
Co-authored-by: Cursor <cursoragent@cursor.com>
* Tests: cover antigravity model ID normalization
* Changelog: note antigravity pro tier normalization
* Tests: type antigravity model helper inputs
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Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Vincent Koc <vincentkoc@ieee.org>
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