refactor: heart rate computationRevert and test coverage

• Refactor EcgFeatures::ComputeHeartRate():

• Restore average RR interval calculation using last 8 peaks.

• Revert validation order and buffer size to previous version.

• Test suite improvements:

  • Rename sampling_rate to waveform_fs_hz across all test files for clarity.
  • Add new test fixtures for BIDMC datasets (bidmc01, bidmc02) with baseline removal.
  • Extend sel103_extracted_feature_test with comprehensive ST/QT wave validation, QRS area comparison, and amplitude/duration checks against reference annotations.
  • Remove obsolete sel103_wfdb_heart_rate_validation_test.
  • Increase MAX_ALLOWED_SAMPLES to 10000 to support longer records.