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/providers/registry/gemini.js

@@ -36,6 +36,7 @@ export default {
     },
   },
   models: [
+    { id: "gemini-3.7-flash", name: "Gemini 3.7 Flash" },
     { id: "gemini-3.6-flash", name: "Gemini 3.6 Flash" },
     { id: "gemini-3.5-flash-lite", name: "Gemini 3.5 Flash Lite" },
     { id: "gemini-3.1-pro-preview", name: "Gemini 3.1 Pro Preview" },