fix(oauth): align antigravity OAuth metadata with official client headers

Fixes #1226

The Antigravity OAuth flow sent inconsistent client metadata between
the token acquisition phase and the API usage phase. String enum values
(IDE_UNSPECIFIED, PLATFORM_UNSPECIFIED) were used during OAuth token
exchange + loadCodeAssist + onboardUser, while numeric enums (ideType: 9,
platform: <computed>, pluginType: 2) were used in runtime API calls.
Google detected this fingerprint mismatch and blocked 9router accounts.

Replace all string enum occurrences with the correct numeric values:

- src/lib/oauth/constants/oauth.js: loadCodeAssistClientMetadata now
  uses getOAuthPlatformEnum() for platform and numeric 9/2 for
  ideType/pluginType, matching getOAuthClientMetadata()
- src/lib/oauth/services/antigravity.js: getMetadata() now delegates
  to getOAuthClientMetadata() instead of returning hardcoded strings
- src/lib/oauth/providers.js: postExchange metadata now uses
  getOAuthClientMetadata() instead of inline string enums
- open-sse/services/usage.js: getGeminiSubscriptionInfo body now uses
  CLIENT_METADATA (already imported from appConstants.js) instead of
  inline string enums

src/lib/oauth/providers.js

@@ -23,6 +23,7 @@ import {
   CLINE_CONFIG,
   GITLAB_CONFIG,
   CODEBUDDY_CONFIG,
+  getOAuthClientMetadata,
 } from "./constants/oauth";
 
 const BASE64_BLOCK_SIZE = 4;
@@ -306,7 +307,7 @@ const PROVIDERS = {
       return await response.json();
     },
     postExchange: async (tokens) => {
-      // Matches CLIProxyAPI Go source: string enum, no mode field
+      // Numeric enums matching Antigravity binary ClientMetadata
       const loadHeaders = {
         "Authorization": `Bearer ${tokens.access_token}`,
         "Content-Type": "application/json",
@@ -315,7 +316,7 @@ const PROVIDERS = {
         "Client-Metadata": ANTIGRAVITY_CONFIG.loadCodeAssistClientMetadata,
         "x-request-source": "local",
       };
-      const metadata = { ideType: "IDE_UNSPECIFIED", platform: "PLATFORM_UNSPECIFIED", pluginType: "GEMINI" };
+      const metadata = getOAuthClientMetadata();
 
       // Fetch user info
       const userInfoRes = await fetch(`${ANTIGRAVITY_CONFIG.userInfoUrl}?alt=json`, {