feat(gemini): add Gemini 3.6 Flash tier routing and 3.5 Flash Lite

Add gemini-3.6-flash tiered (high/medium/low) for Antigravity routing
via upstreamModelId "gemini-3.6-flash-tiered(level)" + thinkingLevel,
plus gemini-3.6-flash and gemini-3.5-flash-lite direct API models.

- getModelUpstreamId: split (level) suffix before lookup, re-append after
- Antigravity executor: preserve transformed body.model
- MITM extractModel: parse thinkingLevel for tiered model (default medium)
- Isolate Cloud Code endpoints: discovery (loadCodeAssist/onboardUser/
  quota) on PROD cloudcode-pa, chat transport on daily-cloudcode-pa
  to bypass prod 429

open-sse/config/appConstants.js

@@ -134,10 +134,19 @@ export const ANTIGRAVITY_HEADERS = {
   "User-Agent": ANTIGRAVITY_IDE_USER_AGENT
 };
 
-// Cloud Code Assist API
+// Cloud Code Assist API endpoints differ by client ecosystem.
 export const CLOUD_CODE_API = {
-  loadCodeAssist: "https://cloudcode-pa.googleapis.com/v1internal:loadCodeAssist",
-  onboardUser: "https://cloudcode-pa.googleapis.com/v1internal:onboardUser",
+  "gemini-cli": {
+    loadCodeAssist: "https://cloudcode-pa.googleapis.com/v1internal:loadCodeAssist",
+    onboardUser: "https://cloudcode-pa.googleapis.com/v1internal:onboardUser",
+  },
+  // Project discovery (loadCodeAssist/onboardUser) stays on PROD — the daily host
+  // rejects these auth/onboarding calls. Only chat traffic uses the daily host
+  // (see transport.apiEndpoint in registry/antigravity.js, set to bypass prod 429).
+  antigravity: {
+    loadCodeAssist: "https://cloudcode-pa.googleapis.com/v1internal:loadCodeAssist",
+    onboardUser: "https://cloudcode-pa.googleapis.com/v1internal:onboardUser",
+  },
 };
 
 export const LOAD_CODE_ASSIST_HEADERS = {