fix(antigravity): bump client version to 1.25.0 for gemini-3.1 model access

[manaporkun]

Summary

• Bumps antigravityVersion from 1.15.8 to 1.25.0 in the Antigravity provider
• Without this, Google's Cloud Code Assist API rejects newer models (gemini-3.1-pro-low, gemini-3.1-flash-lite, etc.) — the model itself responds with "Gemini 3.1 Pro is not available on this version. Please upgrade to the latest version."
• The API checks the User-Agent header (antigravity/<version>) and gates model access behind minimum client versions

Root cause

pkg/providers/antigravity_provider.go:24 — the version string 1.15.8 is too old for gemini-3.1 models. Bumping to 1.25.0 (matching current Antigravity IDE releases) resolves the issue.

Test plan

• Verified gemini-3.1-pro-low returns "not available" with version 1.15.8
• Verified gemini-3.1-pro-low works correctly with version 1.25.0 (tested via direct API call and patched binary)
• gemini-3-flash and other existing models continue to work

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the Antigravity (Google Cloud Code Assist) provider’s client version so requests identify as a newer antigravity/<version> client, which is required for access to newer Gemini 3.1 models.

Changes:

• Bump antigravityVersion from 1.15.8 to 1.25.0 to satisfy the API’s minimum client version gating.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

antigravityVersion is only used for the Chat request’s User-Agent header, but other Antigravity API calls in this file (e.g., FetchAntigravityProjectID and FetchAntigravityModels) still send User-Agent: antigravity without a version. Given the API gates features/models based on the antigravity/<version> User-Agent, these endpoints may remain blocked or behave inconsistently. Consider centralizing a single versioned User-Agent value and using it for all Antigravity HTTP requests in this provider.