Antigravity: improve OAuth client setup diagnostics

[Yuxin-Qiao]

Summary

Improves the Antigravity OAuth setup diagnostic shown when CodexBar cannot discover the Antigravity OAuth client.

The updated message clarifies that CodexBar expects OAuth client metadata from Antigravity.app or explicit ANTIGRAVITY_OAUTH_CLIENT_ID / ANTIGRAVITY_OAUTH_CLIENT_SECRET environment variables, and notes that the Antigravity CLI alone may not provide the client metadata needed by CodexBar after the Gemini CLI migration.

Scope

• Antigravity only
• diagnostics/message only
• no auth behavior changes
• no quota parsing changes
• no Gemini provider changes
• no CLI discovery changes

Testing

• swift test --filter AntigravityOAuthCredentialsStoreTests
• make check
• git diff --check

[Yuxin-Qiao]

Closing this since #1090 has landed and fixes the root Antigravity OAuth client discovery path for #1076.

This PR was intentionally narrower and only improved the fallback diagnostic when OAuth client discovery fails. With the root discovery fix merged, I’m happy to keep this closed unless a clearer diagnostic is still needed later.

Thanks!