feat(antigravity): add Gemini 3.7 Flash models (#3286, #3281)

Add gemini-3.7-flash and its tiered high/medium/low variants to the
Antigravity and Gemini registries, with matching capabilities, pricing
and Antigravity quota tracking.

extractModel now recognises gemini-3.7-flash-tiered alongside 3.6 and
derives the version from the request, so thinkingLevel still maps to the
right tiered alias.

Closes #3286
Closes #3281

open-sse/services/usage/google.js

@@ -161,6 +161,9 @@ export async function getAntigravityUsage(accessToken, providerSpecificData, pro
     if (data.models) {
       // Filter only recommended/important models (must match PROVIDER_MODELS ag ids)
       const importantModels = [
+        'gemini-3.7-flash-high',
+        'gemini-3.7-flash-medium',
+        'gemini-3.7-flash-low',
         'gemini-3.6-flash-high',
         'gemini-3.6-flash-medium',
         'gemini-3.6-flash-low',