test: add respiration rate validation for BIDMC02 and fix setup

• Add bidmc02_RespirationRateValidationTest to validate derived respiration against reference.
• Fix setup_ecg_bidmc02_file:
  • Correct signal count from 6 to 5.
  • Read ECG from lead index 4 (was 1) and respiration from index 0.
  • Apply median filtering to remove baseline from ECG and respiration signals.
  • Use "breath" annotation file instead of "atr".
  • Store baseline-removed signals for further use.