fix(oauth): scope antigravity header fixes to loadCodeAssist/onboardUser only

Google fingerprints User-Agent/Client-Metadata on loadCodeAssist and
onboardUser, silently refusing to provision a cloudaicompanionProject
when they don't match the real IDE. Split antigravity's headers out of
the shared gemini-cli constants instead of overwriting them, so the fix
doesn't touch gemini-cli or any other provider.

Inspired by #3000 (thanks @stoXmod for flagging the resource-exhausted
issue), rewritten to keep gemini-cli untouched.

open-sse/providers/registry/antigravity.js

@@ -76,8 +76,7 @@ export default {
     apiVersion: "v1internal",
     loadCodeAssistEndpoint: "https://cloudcode-pa.googleapis.com/v1internal:loadCodeAssist",
     onboardUserEndpoint: "https://cloudcode-pa.googleapis.com/v1internal:onboardUser",
-    loadCodeAssistUserAgent: "google-api-nodejs-client/9.15.1",
-    loadCodeAssistApiClient: "google-cloud-sdk vscode_cloudshelleditor/0.1",
+    loadCodeAssistUserAgent: ANTIGRAVITY_IDE_USER_AGENT,
     refreshLeadMs: 300000,
   },
   features: {