[Test restructure 4] Reclassify misplaced tests at the unit/integration boundary

## What

Handle edge-case files that are currently in the wrong tier.

## Files to investigate and reclassify

| File | Current location | Issue | Proposed action |
|------|-----------------|-------|-----------------|
| `test_mcp_feature_server.py` | `integration/` | Fully mocked, no external services, no markers | Move to `unit/` |
| `test_rest_error_decorator.py` | `unit/` | Uses `environment` fixture with no markers | Investigate: refactor to remove `environment` dep (preferred) or move to `universal/` |
| `test_offline_server.py` | `unit/` | Partially uses `environment` | Split: env-dependent tests → `universal/`, rest stays in `unit/` |
| `test_auth_registry_server.py` | `unit/permissions/auth/server/` | Starts real HTTP server with TLS | Move to `integration/auth/` or `component/` |
| `test_dbt_integration.py` | `integration/dbt/` | Missing `@pytest.mark.integration` — runs during unit CI | Will be fixed structurally by living in `integration/dbt/` |

## Note

Each file may need individual judgment. The goal is zero tests that straddle tier boundaries after this PR.