support patternProperties required in schema

[braingram]

This updates the from-schema building code to handle patternProperties to:

• allow possibly adding required to some reference files (see Update the reference file schemas to use explicit entries rad#712)
• drop WfiImgPhotomRefMixin which existed only to work around our lack of patternProperties support

Regtests all pass: https://github.com/spacetelescope/RegressionTests/actions/runs/18477730458

Tasks

• Update or add relevant roman_datamodels 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).
  • Start a romancal regression test (https://github.com/spacetelescope/RegressionTests/actions/workflows/romancal.yml) with this branch installed ("git+https://github.com/<fork>/rad@<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

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.84%. Comparing base (087a60d) to head (3496b1f).
⚠️ Report is 222 commits behind head on main.

Files with missing lines	Patch %	Lines
src/roman_datamodels/_stnode/_schema.py	90.90%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #591      +/-   ##
==========================================
- Coverage   97.56%   94.84%   -2.72%     
==========================================
  Files          30       27       -3     
  Lines        2788     2677     -111     
==========================================
- Hits         2720     2539     -181     
- Misses         68      138      +70     

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