[Fix] Calculate RSP phase durations from sample positions

[bipin-vishwakarma]

Summary

• calculate respiratory phase durations from row/sample positions instead of DataFrame index labels
• preserve existing behavior for continuous signals while fixing time-indexed epochs
• add a regression test comparing continuous and epoched representations

Why

epochs_create() uses time values for the DataFrame index. Treating those labels as sample numbers and dividing by sampling_rate scales phase durations down a second time.

Fixes #1196.

Tests

• pytest tests/tests_rsp.py -k "rsp_intervalrelated" -q -o addopts="" (2 passed)
• complete tests/tests_rsp.py module: 25 passed across the regular run and the report test rerun with MPLBACKEND=Agg (local Python lacks Tk data)
• ruff check neurokit2/rsp/rsp_intervalrelated.py tests/tests_rsp.py
• ruff format --check neurokit2/rsp/rsp_intervalrelated.py tests/tests_rsp.py

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.17%. Comparing base (9f3a1a5) to head (64dbbc8).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1201   +/-   ##
=======================================
  Coverage   58.17%   58.17%           
=======================================
  Files         310      310           
  Lines       15702    15703    +1     
=======================================
+ Hits         9135     9136    +1     
  Misses       6567     6567           

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