The Antigravity service fingerprints clients by the version embedded in the User-Agent header. The built-in default (1.23.2) had drifted far behind current real client builds, so requests using the default UA started failing OAuth and probe flows.
Bump DefaultUserAgentVersion from 1.23.2 to 2.9.1 (current client build) and update the constant test accordingly. The default remains overridable via ANTIGRAVITY_USER_AGENT_VERSION and the admin setting.
The ClientSecret constant was left as an empty string after
getClientSecret() was refactored to use defaultClientSecret.
Remove the dead constant and update the test accordingly.
- Fix oauth_test.go and client_test.go to use defaultClientSecret
variable instead of env var (init() already sets the default)
- Align migration 058 gemini-3-pro-high/low/preview mappings with
constants.go (map to 3.1 versions)