- 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)
Add migration 058 to update existing Antigravity accounts with
claude-sonnet-4-6 in model_mapping. Add migration 059 to add
gemini-3.1-pro-high/low/preview mappings.