switch check in elp to use result instead of model

[braingram]

While working on some related changes I noticed that the elp is accessing model (the input model, which may be closed) instead of result when checking the source_catalog sub-tree. This PR switches the access to use result. I don't think by itself the code on main can cause an issue with current asdf and roman_datamodels but it seems more correct to use result (and if we change how roman_datamodels handles a closed asdf file the code on main could be problematic).

Regression tests:
https://github.com/spacetelescope/RegressionTests/actions/runs/29345171004/job/87126875995

Tasks

• request a review from someone specific, to avoid making the maintainers review every PR
• add a build milestone, i.e. 24Q4_B15 (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 existing functionality, 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.82%. Comparing base (35c3dc2) to head (b565a0c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2389   +/-   ##
=======================================
  Coverage   82.82%   82.82%           
=======================================
  Files         131      131           
  Lines        7249     7249           
=======================================
  Hits         6004     6004           
  Misses       1245     1245           

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[schlafly]

Good catch, thanks Brett.