JP-3426: Check WFSS background mask against input dq array

[tapastro]

Resolves JP-3426

This PR addresses errors seen in operations processing of a WFSS program - the background subtraction step generates a mask by excluding pixels deemed to have source flux using bounding boxes. In the case of the problematic dataset, the only pixels remaining after source exclusion is the 4-pixel wide boundary corresponding to reference pixels. This PR adds an additional check by masking the input model DQ array and ensuring the remaining pixels do not have DO_NOT_USE flagged.

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

[tapastro]

Made an edit to addressing dqflags values rather than assuming flag of interest is 1, restarting the regtest run.

Regtests here: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1051/

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (9dfaa37) 75.95% compared to head (0480752) 75.95%.

Files	Patch %	Lines
jwst/background/background_sub.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8070      +/-   ##
==========================================
- Coverage   75.95%   75.95%   -0.01%     
==========================================
  Files         459      459              
  Lines       37591    37592       +1     
==========================================
  Hits        28552    28552              
- Misses       9039     9040       +1     

Flag	Coverage Δ		*Carryforward flag
nightly	77.37% <ø> (-0.01%)	⬇️	Carriedforward from 9dfaa37

*This pull request uses carry forward flags. Click here to find out more.

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[hbushouse]

No regtest failures and CI's look good. I'm merging.