* feat: add modelId fallback for provider validation
- If /models endpoint unavailable, validate via /chat/completions
- Add optional Model ID input in EditCompatibleNodeModal
- Improves compatibility with providers lacking /models endpoint
* feat: improve provider validation with modelId fallback
- Add Model ID input for chat/completions fallback validation
- Reorder UI: API Key → Model ID → Check button + Badge
- Display detailed BE error messages in FE
- Add status-specific error handling (401/403/400/404/5xx)
- Add unit tests for error message helpers
- Add vitest devDependency
