JP-3859: Style updates for flat_field

[melanieclarke]

Resolves JP-3859

Style and documentation fixes for the flat field step.

Tasks

• If you have a specific reviewer in mind, tag them.
• add a build milestone, i.e. Build 12.0 (use the latest build if not sure)
• Does this PR change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see changelog readme for instructions)
  • update or add relevant tests
  • update relevant docstrings and / or docs/ page
  • start a regression test and include a link to the running job (click here for instructions)
    • Do truth files need to be updated ("okified")?
      • after the reviewer has approved these changes, run okify_regtests to update the truth files
• if a JIRA ticket exists, make sure it is resolved properly

[melanieclarke]

Regtests here: https://github.com/spacetelescope/RegressionTests/actions/runs/15031169793

Tests are passing.

[emolter]

Looks great Melanie, thanks! I left a few small comments but TBH despite the +-850 code lines I was unable to find much, including nitpicks

[codecov]

Codecov Report

❌ Patch coverage is 60.78431% with 80 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.90%. Comparing base (63fe3b5) to head (f9de497).
⚠️ Report is 657 commits behind head on main.

Files with missing lines	Patch %	Lines
jwst/flatfield/flat_field.py	59.23%	75 Missing ⚠️
jwst/flatfield/flat_field_step.py	73.68%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9464      +/-   ##
==========================================
+ Coverage   77.80%   77.90%   +0.10%     
==========================================
  Files         363      362       -1     
  Lines       36388    36334      -54     
==========================================
- Hits        28311    28307       -4     
+ Misses       8077     8027      -50     

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