JP-3890: One missing enum value for DHS subarray

[tapastro]

Resolves JP-3890
Matches work in JWSTKD-566

This PR addresses a subarray value missed in updates for JP-3890.

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

[braingram]

Does this also need to go into subarray.schema and p_subarray.schema?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.35%. Comparing base (088e8a6) to head (2954391).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #428   +/-   ##
=======================================
  Coverage   78.35%   78.35%           
=======================================
  Files         109      109           
  Lines        5129     5129           
=======================================
  Hits         4019     4019           
  Misses       1110     1110           

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[braingram]

LGTM.