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

tests/__baseline__/known-fails.txt

@@ -1,4 +1,3 @@
-tests/unit/antigravity-mitm.test.js :: Antigravity MITM model handling flags the out-of-box agent/Default model mandatory
 tests/unit/claude-header-forwarding.test.js :: proxyAwareFetch — api.anthropic.com routing routes api.anthropic.com to gotScraping (non-streaming) and returns ok response
 tests/unit/oauth-cursor-auto-import.test.js :: GET /api/oauth/cursor/auto-import extracts tokens using exact keys
 tests/unit/oauth-cursor-auto-import.test.js :: GET /api/oauth/cursor/auto-import falls back to fuzzy key matching on macOS when exact keys are missing

tests/__baseline__/providers-baseline.json

@@ -25,7 +25,7 @@
   },
   "antigravity": {
     "baseUrls": [
-      "https://cloudcode-pa.googleapis.com"
+      "https://daily-cloudcode-pa.googleapis.com"
     ],
     "format": "antigravity",
     "headers": {
@@ -881,4 +881,4 @@
     },
     "format": "openai"
   }
-}
+}