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.

src/lib/oauth/providers/antigravity.js

@@ -39,13 +39,10 @@ const antigravity = {
     return await response.json();
   },
   postExchange: async (tokens) => {
-    // Numeric enums matching Antigravity binary ClientMetadata
     const loadHeaders = {
       "Authorization": `Bearer ${tokens.access_token}`,
       "Content-Type": "application/json",
       "User-Agent": ANTIGRAVITY_CONFIG.loadCodeAssistUserAgent,
-      "X-Goog-Api-Client": ANTIGRAVITY_CONFIG.loadCodeAssistApiClient,
-      "Client-Metadata": ANTIGRAVITY_CONFIG.loadCodeAssistClientMetadata,
       "x-request-source": "local",
     };
     const metadata = getOAuthClientMetadata();

src/lib/oauth/services/antigravity.js

@@ -85,8 +85,6 @@ export class AntigravityService {
       "Authorization": `Bearer ${accessToken}`,
       "Content-Type": "application/json",
       "User-Agent": this.config.loadCodeAssistUserAgent,
-      "X-Goog-Api-Client": this.config.loadCodeAssistApiClient,
-      "Client-Metadata": this.config.loadCodeAssistClientMetadata,
     };
   }