Backport PR #9507: JP-4021: Fix a bug where insufficient valid background pixels case was not caught for WFSS background step

[zacharyburnett]

backporting #9507

regtest: https://github.com/spacetelescope/RegressionTests/actions/runs/15540520998/job/43750089133

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/1.18.x@699bda8). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
jwst/background/background_sub_wfss.py	66.66%	4 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##             release/1.18.x    #9528   +/-   ##
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  Coverage                  ?   74.60%           
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  Files                     ?      369           
  Lines                     ?    37163           
  Branches                  ?        0           
=================================================
  Hits                      ?    27727           
  Misses                    ?     9436           
  Partials                  ?        0           

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[zacharyburnett]

new regtest against 1.18.0 and jwst_1364.pmap: https://github.com/spacetelescope/RegressionTests/actions/runs/15542519446/job/43756624199

[zacharyburnett]

the only test NEWLY failing (not also failing on main) is jwst.regtest.test_niriss_wfss::test_nis_wfss_spec2[extract_2d]:
https://github.com/spacetelescope/RegressionTests/actions/runs/15542519446/attempts/1#summary-43756624199