Adds TestOIDCEnvVarsWithNilConfig to catch the case where OIDC_* env
vars were silently ignored when no oidc.enc file existed. This documents
the proper pattern of initializing OIDCConfig before calling MergeFromEnv.
Test coverage for internal/config/oidc.go functions:
- normaliseList: 8 cases for deduplication, trimming, empty handling
- parseDelimited: 8 cases for comma/space separation
- DefaultRedirectURL: 7 cases for URL construction
- OIDCConfig.Clone: 2 cases for deep copy behavior
- OIDCConfig.ApplyDefaults: 9 cases for defaults and normalization
- OIDCConfig.Validate: 9 cases for validation rules
- OIDCConfig.MergeFromEnv: 5 cases for environment variable merging
- NewOIDCConfig: constructor verification
Total: 56 new test cases (583 lines).
