- Change auth config from provider_type + config dict to discriminated union types
- Add GitHub token authentication provider
- Improve auth error messages with provider-specific guidance
- Extract auth datatypes to separate module
- Update tests to use new auth config structure
- Remove unused OAuth2LocalJWTConfig
## Test Plan
- Unit tests pass for all auth providers
- Integration tests verify auth flow works correctly
- GitHub token auth tested with valid/invalid tokens
Co-Authored-By: Claude <noreply@anthropic.com>
