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

open-sse/services/usage.js

@@ -305,11 +305,7 @@ async function getGeminiSubscriptionInfo(accessToken, proxyOptions = null) {
           "Content-Type": "application/json",
         },
         body: JSON.stringify({
-          metadata: {
-            ideType: "IDE_UNSPECIFIED",
-            platform: "PLATFORM_UNSPECIFIED",
-            pluginType: "GEMINI",
-          },
+          metadata: CLIENT_METADATA,
         }),
         signal: controller.signal,
       },