Make all $ref absolute URI refs

[WilliamJamieson]

This PR makes all $ref uris absolute URIs (the id registered for a given schema in ASDF) rather than relative URIs. Additionally it adds tests which enforce this convention.

This is useful for two reasons:

1. It makes all the URI usage (both tag and $ref) follow a consistent pattern.
2. Makes processing of $ref values outside of the directory structure of rad or ASDF easier as it is much easier to look up based on the id URI rather than figuring out a $ref based on context.

Note this breaks one test in roman_datamodels, which for some reason is testing RAD schema contents rather than datamodels (it checks ref_common is included in all reference file schemas).

Tasks

• Update or add relevant rad tests.
• Update relevant docstrings and / or docs/ page.
• Does this PR change any schema files?
  • Schema changes were discussed at RAD Review Board meeting.
• 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>").
  • Update relevant roman_datamodels utilities and tests.

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 96.59%. Comparing base (0fd380a) to head (d96b8da).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #527      +/-   ##
==========================================
+ Coverage   96.41%   96.59%   +0.18%     
==========================================
  Files           4        4              
  Lines         223      235      +12     
==========================================
+ Hits          215      227      +12     
  Misses          8        8              

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[WilliamJamieson]

Regtests: https://github.com/spacetelescope/RegressionTests/actions/runs/12552766063, failing matches those failing on main: https://github.com/spacetelescope/RegressionTests/actions/runs/12553473664