WIP: more cleanup

[braingram]

• remove _check_args and DISCOURAGED_TYPES
• duck-typing in save

jwst regtests: https://github.com/spacetelescope/RegressionTests/actions/runs/11166836974
romancal regtests: https://github.com/spacetelescope/RegressionTests/actions/runs/11168353215

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 73.52%. Comparing base (5efae61) to head (f512afb).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/stpipe/step.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
+ Coverage   73.29%   73.52%   +0.23%     
==========================================
  Files          25       25              
  Lines        1917     1896      -21     
==========================================
- Hits         1405     1394      -11     
+ Misses        512      502      -10     

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