fix(tests): ensure proper test isolation and cleanup

fix(tests): ensure proper test isolation and cleanup

Add afterEach hooks to reset singleton state and clear factory caches
between tests, preventing state leakage across test files.

Changes:
- OCPP20VariableManager.test.ts: Reset runtime overrides after each test
- OCPPAuthServiceFactory.test.ts: Clear all cached instances after each test
- OCPP20IncomingRequestService-SetVariables.test.ts: Reset variable manager
- OCPP20IncomingRequestService-GetVariables.test.ts: Reset variable manager
- OCPP20IncomingRequestService-GetBaseReport.test.ts: Reset variable manager

Verified: All 280 tests pass on consecutive runs (test && test)