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>

src/shared/constants/cliTools.js

@@ -8,7 +8,7 @@ export const MITM_TOOLS = {
     description: "Google Antigravity IDE with MITM",
     configType: "mitm",
     mitmDomain: "daily-cloudcode-pa.googleapis.com",
-    modelAliases: ["gemini-3.5-flash-low", "gemini-3-flash-agent", "gemini-3.5-flash-extra-low", "gemini-3.1-pro-low", "gemini-pro-agent", "claude-sonnet-4-6", "claude-opus-4-6-thinking", "gpt-oss-120b-medium", "gemini-3-flash"],
+    modelAliases: ["gemini-3-flash-agent", "gemini-3.5-flash-low", "gemini-3.5-flash-extra-low", "gemini-3.1-pro-low", "gemini-pro-agent", "claude-sonnet-4-6", "claude-opus-4-6-thinking", "gpt-oss-120b-medium", "gemini-3-flash"],
     defaultModels: [
       { id: "gemini-3.5-flash-low", name: "Gemini 3.5 Flash (Medium) / Default", alias: "gemini-3.5-flash-low" },
       { id: "gemini-3-flash-agent", name: "Gemini 3.5 Flash (High)", alias: "gemini-3-flash-agent" },