JP-4169: Allow source splitting across chunks in wfss_contam

[emolter]

Resolves JP-4169

Closes #9955

This PR allows large sources to be split across chunks of size max_pixels_per_chunk, and also enforces that all chunks except the last chunk be of exactly size max_pixels_per_chunk.

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

[emolter]

https://github.com/spacetelescope/RegressionTests/actions/runs/18787364832

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.52%. Comparing base (bbf1e5f) to head (c78c1e1).
⚠️ Report is 67 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9956      +/-   ##
==========================================
+ Coverage   85.98%   86.52%   +0.54%     
==========================================
  Files         368      366       -2     
  Lines       38457    38479      +22     
==========================================
+ Hits        33066    33294     +228     
+ Misses       5391     5185     -206     

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