feat: enable SignalTest and add slf4j-simple for test logging

- Applied SignalsPlugin fix from bcb43396b (getIdentifier() ?? add() pattern)
- Converted SignalTest to use AbstractFullStackTest
- Created SignalEndpoint.ts snapshot with correct type imports
- Added slf4j-simple test dependency to eliminate SLF4J warnings
- Updated full-stack-test.md progress (5/47 tests converted)

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Co-Authored-By: Claude <[email protected]>

Author: Artur-