feat: Detect Antigravity IDE for usage tracking

[virrrat]

Tested:

[gemini-code-assist]

Code Review

This pull request introduces an is_antigravity environment detection function and integrates it into the get_client_environment_label utility. Feedback highlights the need to update the get_client_environment_label docstring to include the new environment in the priority list. Additionally, existing tests for environment labeling should be updated to mock the new detector to maintain isolation, and a new test case should be added to verify the 'antigravity' label mapping.