refactor(tests): use testable interfaces in OCPP 2.0 tests (Wave 2)

refactor(tests): use testable interfaces in OCPP 2.0 tests (Wave 2)

Refactored 15 OCPP 2.0 test files to use type-safe testable interfaces:

IncomingRequestService tests (11 files):
- Reset, ClearCache, CertificateSigned, GetBaseReport, GetVariables
- SetVariables, RequestStartTransaction, RequestStopTransaction
- InstallCertificate, DeleteCertificate, GetInstalledCertificateIds

RequestService tests (2 files):
- ISO15118, SignCertificate

VariableManager test (1 file):
- VariableManager

Created testable interfaces:
- TestableOCPP20IncomingRequestService (11 handler methods)
- TestableOCPP20RequestService (sendMessage mocking)
- TestableOCPP20VariableManager (2 validation methods)

Impact:
- Eliminated ~200 'as any' casts across 15 files
- Removed ~50 eslint-disable directives
- Full type safety with IntelliSense support
- All 280 tests passing

Note: 48 lint errors remain (import ordering, unsafe assignments).
Will address in follow-up commit.