feat(antigravity): add gemini-3.5-flash-extra-low (Low) model

- Add gemini-3.5-flash-extra-low across CLI menu, provider models, usage, pricing
- Add MITM synonyms (high/medium/extra-low) and split pattern so Low no longer falls through to Medium
- Strip models/ prefix in getMappedModel for AG public name normalization

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/services/usage.js

@@ -399,6 +399,7 @@ async function getAntigravityUsage(accessToken, providerSpecificData, proxyOptio
       const importantModels = [
         'gemini-3-flash-agent',
         'gemini-3.5-flash-low',
+        'gemini-3.5-flash-extra-low',
         'gemini-pro-agent',
         'gemini-3.1-pro-low',
         'claude-sonnet-4-6',