JP-4258: Polynomial fitting of simulated source SEDs for wfss_contam

[emolter]

Resolves JP-4258

This PR adds optional spectral fitting to the wfss_contam step.

The basic setup is intended to mimic what's done by the GRIZLI code. See the new docs pages and docstrings for a short description of how it works. See the ticket for some figures that demonstrate the capability.

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)
    • if your change breaks step-level or public API (as defined in the docs), also add a changes/<PR#>.breaking.rst news fragment
  • 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

[codecov]

Codecov Report

❌ Patch coverage is 95.52239% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.21%. Comparing base (745bf6b) to head (1a87293).

Files with missing lines	Patch %	Lines
jwst/wfss_contam/wfss_contam.py	95.49%	5 Missing ⚠️
jwst/wfss_contam/wavefit.py	92.15%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10458      +/-   ##
==========================================
+ Coverage   86.18%   86.21%   +0.03%     
==========================================
  Files         372      373       +1     
  Lines       40103    40256     +153     
==========================================
+ Hits        34564    34708     +144     
- Misses       5539     5548       +9     

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[emolter]

initial regression test round: https://github.com/spacetelescope/RegressionTests/actions/runs/25075045516

All passing, which is expected because this is not supposed to affect the case without PSF modeling.