test: update ECG validation and heart rate computation
- Replace sampling_rate usage with waveform_fs_hz in WFDB tests
- Disable ehemhr250 and del103hr validation tests temporarily
- Update SEL103 R-peak validation and BPM tolerance
- Improve ComputeHeartRate RR interval validation
- Add rolling average RR-based heart rate calculation
- Retain latest 9 R peaks for 8 RR intervals
- Add unit tests for invalid RR configuration and HR calculation
- Update QRS validation count and reference lead handling