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)
