avoid an extra validation on __init__

[braingram]

When a SlitModel is created from a MultiSlitModel.slits[i].instance as is done in pixel_replace:
https://github.com/spacetelescope/jwst/blob/a32f8b01e8000603d6cc916511a87c0e0b77e47e/jwst/pixel_replace/pixel_replace.py#L187

The instance may contain the int_times FITS_rec and can lead to an AsdfConversionWarning (that will soon be an error asdf-format/asdf#1858):
https://github.com/spacetelescope/RegressionTests/actions/runs/11632063641/job/32394552214#step:28:266

This PR changes Model.__init__ to avoid the validation on __init__ (which is also deprecated and soon be removed: asdf-format/asdf#1852). This avoids the AsdfConversionWarning (due to an attempt to serialize a FITS_rec ) and has the added benefit of avoiding an unnecessary validation (the data will be validated on write where the FITS_rec will be safely converted).

Regression tests: https://github.com/spacetelescope/RegressionTests/actions/runs/11666451738
all pass and most relevant to this PR show no AsdfConversionWarning.

This change is similar to #230 in that it uses assignment to the private _tree attribute to avoid validation.

Tasks

• update or add relevant tests
• update relevant docstrings and / or docs/ page
• Does this PR change any API used downstream? (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)
  • run jwst regression tests with this branch installed ("git+https://github.com/<fork>/stdatamodels@<branch>")

news fragment change types...

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: fixes an issue
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.misc.rst: infrastructure or miscellaneous change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.52%. Comparing base (a6994fa) to head (66d8d0a).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #356      +/-   ##
==========================================
+ Coverage   67.51%   67.52%   +0.01%     
==========================================
  Files         114      114              
  Lines        5913     5916       +3     
==========================================
+ Hits         3992     3995       +3     
  Misses       1921     1921              

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[tapastro]

LGTM