- Add OIDC_ENABLED environment variable to control OIDC authentication
- Validate required OIDC env vars (OIDC_ISSUER, OIDC_CLIENT_ID, OIDC_CLIENT_SECRET)
are present when OIDC is enabled
- Validate OIDC_ISSUER ends with trailing slash for correct discovery URL
- Throw descriptive error at startup if configuration is invalid
- Skip OIDC plugin registration when OIDC is disabled
- Add comprehensive tests for validation logic (17 test cases)
Refs #337
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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