RCAL-992: only single json encode skycell wcs in asn

[braingram]

Resolves RCAL-992

Closes #1589

This PR fixes the bug where the skycell_wcs_info contents added by skycell_asn to the association files is double json encoded.

Regtests https://github.com/spacetelescope/RegressionTests/actions/runs/12991881301

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 below for change types)
  • 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

news fragment change types...

• changes/<PR#>.general.rst: infrastructure or miscellaneous change
• changes/<PR#>.docs.rst
• changes/<PR#>.stpipe.rst
• changes/<PR#>.associations.rst
• changes/<PR#>.scripts.rst
• changes/<PR#>.mosaic_pipeline.rst
• changes/<PR#>.patch_match.rst

steps

• changes/<PR#>.dq_init.rst
• changes/<PR#>.saturation.rst
• changes/<PR#>.refpix.rst
• changes/<PR#>.linearity.rst
• changes/<PR#>.dark_current.rst
• changes/<PR#>.jump_detection.rst
• changes/<PR#>.ramp_fitting.rst
• changes/<PR#>.assign_wcs.rst
• changes/<PR#>.flatfield.rst
• changes/<PR#>.photom.rst
• changes/<PR#>.flux.rst
• changes/<PR#>.source_detection.rst
• changes/<PR#>.tweakreg.rst
• changes/<PR#>.skymatch.rst
• changes/<PR#>.outlier_detection.rst
• changes/<PR#>.resample.rst
• changes/<PR#>.source_catalog.rst

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.94%. Comparing base (262df9a) to head (f12325b).

Files with missing lines	Patch %	Lines
romancal/pipeline/mosaic_pipeline.py	0.00%	2 Missing ⚠️
romancal/associations/skycell_asn.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1591   +/-   ##
=======================================
  Coverage   77.93%   77.94%           
=======================================
  Files         115      115           
  Lines        7624     7621    -3     
=======================================
- Hits         5942     5940    -2     
+ Misses       1682     1681    -1     

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[braingram]

The association schema is unused and defines a structure that differs from what's expected by the mosaic pipeline. It probably makes sense to address the other schema/structure issues first.