Update tag handling and clean up Builder API

[WilliamJamieson]

There has been something about how stnode objects handle the setting and removal of the tags that ASDF (or users) can attach to nodes that has been bothering me for a long time, see

roman_datamodels/src/roman_datamodels/_stnode/_converters.py

Line 69 in 649c8eb

# TODO: Add method for setting read_tag with some checks

.

This PR attempts to resolve this by adding a constructor to the nodes that takes care of this issue. This in turn means that all the code of the form

if tag:
    new._read_tag = tag

return new

could be rolled together. Note that while fixing this issue I noticed/found some small issues with the Builder/create_* API that could be improved. This PR resolves both the _read_tag and builder API stuff.

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 96.77419% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.33%. Comparing base (087a60d) to head (7e6009d).
⚠️ Report is 255 commits behind head on main.

Files with missing lines	Patch %	Lines
src/roman_datamodels/_stnode/_converters.py	66.66%	1 Missing ⚠️
src/roman_datamodels/_stnode/_registry.py	50.00%	1 Missing ⚠️
src/roman_datamodels/_stnode/_schema.py	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #660      +/-   ##
==========================================
- Coverage   97.56%   95.33%   -2.23%     
==========================================
  Files          30       27       -3     
  Lines        2788     2785       -3     
==========================================
- Hits         2720     2655      -65     
- Misses         68      130      +62     

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[WilliamJamieson]

Regression tests pass: https://github.com/spacetelescope/RegressionTests/actions/runs/25167042077/job/73775874639