fix(test-isolation): move mock instances to beforeEach in OCPP 2.0 tests

fix(test-isolation): move mock instances to beforeEach in OCPP 2.0 tests

Move module-level mock instances inside beforeEach blocks to ensure
proper test isolation per TEST_STYLE_GUIDE.md requirements.

- 11 IncomingRequestService test files fixed
- 1 VariableManager test file fixed
- 1 ServiceUtils-TransactionEvent test file fixed
- 1 TestUtils file updated with JSDoc documentation

Each test now gets fresh mock instances, eliminating potential
cross-test state contamination.