test: add UI dataset read verification and extend WFDB writer tests
- Add UI dataset reader demo for HEM_SAMPLE
- Print WFDB header and signal metadata for verification
- Add sample-by-sample dataset read demonstration
- Add negative test cases for invalid sampling frequency and record name
- Add write_block validation tests (NULL writer, NULL buffer, closed writer, zero frames)
- Refactor writer unit tests and remove redundant cases