test: Add missing error path tests for TCP transport

[groupsky]

Description

TCP transport unit tests have excellent coverage (94.05% statements), but several error paths are not tested.

Missing Unit Tests

1. Write Operations Without Handler

Lines not covered: tcp.ts:174, 214, 250

• setRegister without request handler
• setRegisterArray without request handler
• setCoil without request handler
• getDiscreteInput without request handler

2. Coil and Discrete Input Integration Tests

Missing from emulator.tcp.integration.test.ts:

• Read coils (getCoil)
• Write single coil (setCoil)
• Read discrete inputs (getDiscreteInput)

Expected Coverage Increase

Current: 94.05% statements, 81.57% branches
Target: 95%+ statements, 85%+ branches

Adding 7 tests should cover the missing error paths and bring coverage above 95%.

Acceptance Criteria

• Add 4 unit tests for write operations without handler
• Add 3 integration tests for coil/discrete input operations
• All tests pass
• Coverage increases to 95%+ statements
• Coverage increases to 85%+ branches

Related

• Identified during test coverage review of PR feat(emulator): implement TCP transport #252
• Part of achieving 95% coverage target

[groupsky]

✅ VERIFIED FIXED: tcp.test.ts has comprehensive error path coverage including: dedicated error handling section (line 183), stop during initialization, port binding errors, initialization timeout, stop errors, error event handling, event listener cleanup on errors, and invalid service vector responses.