fix: respect sessions when finding estimators

[mgxd]

find_estimators discarded the caller's sessions whenever any bids_filters were given, because the session key was popped unconditionally and defaulted to None.

Addresses nipreps/fmriprep#3664

This likely warrants a backport to the 2.15.x series(?) for fmriprep LTS

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.13%. Comparing base (6619863) to head (0021399).

Files with missing lines	Patch %	Lines
sdcflows/utils/wrangler.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #554      +/-   ##
==========================================
- Coverage   84.14%   84.13%   -0.01%     
==========================================
  Files          30       30              
  Lines        2939     2938       -1     
  Branches      400      401       +1     
==========================================
- Hits         2473     2472       -1     
  Misses        388      388              
  Partials       78       78              

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