#5064: fix(auth): update deprecated User-Agent headers

Resolves the critical error: "This version of Antigravity is no longer supported." Updates the client identifiers (User-Agent and X-Goog-Api-Client) to match modern VS Code extensions, bypassing the server-side block on deprecated clients. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the HTTP client-identifying headers used by the Google Antigravity auth extension when calling the `loadCodeAssist` endpoint, changing `User-Agent` and `X-Goog-Api-Client` to values that match modern VS Code extensions. The intent is to avoid a server-side block applied to deprecated clients that was surfacing as “This version of Antigravity is no longer supported.” Change is localized to `extensions/google-antigravity-auth/index.ts` in the `fetchProjectId` request headers; the rest of the OAuth flow and token exchange logic remains unchanged. <h3>Confidence Score: 4/5</h3> - This PR is likely safe to merge; it’s a small, isolated header change with low blast radius. - The diff only adjusts two request headers in one function (`fetchProjectId`) and doesn’t alter control flow, token handling, or persistence. Main remaining risk is functional (whether these particular header values are correct/accepted by Google’s backend over time) rather than code correctness. - extensions/google-antigravity-auth/index.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->