JP-4168: Use global wavelength spacing so results don't depend on chunk size for wfss_contam

[emolter]

Resolves JP-4168

Closes #9950

This PR addresses ...

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/18786459070

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.47%. Comparing base (38c180a) to head (51fb32b).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9954      +/-   ##
==========================================
+ Coverage   85.45%   85.47%   +0.02%     
==========================================
  Files         366      366              
  Lines       37845    37904      +59     
==========================================
+ Hits        32341    32400      +59     
  Misses       5504     5504              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.