JP-4228: Update metadata attributes to not assume implicit setting to default on access

[emolter]

Relates to JP-4228

This PR addresses all instances where metadata attributes were assumed to be set to a schema default on access. This change is needed for spacetelescope/stdatamodels#662 but does not break anything so can be handled separately.

There are very few metadata attributes with schema-defined defaults in stdatamodels.
It turns out that this behavior was only meaningfully used by the pipeline for the lamp_mode, asn.exptype, and wcsinfo.s_region attributes.

TODO: there are two explicit attribute sets in combine_1d tests that should be here too

Tasks

• If you have a specific reviewer in mind, tag them.
• add a build milestone, i.e. Build 12.0 (use the latest build if not sure)
• Does this PR change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see changelog readme for instructions)
    • if your change breaks step-level or public API (as defined in the docs), also add a changes/<PR#>.breaking.rst news fragment
  • update or add relevant tests
  • update relevant docstrings and / or docs/ page
  • start a regression test and include a link to the running job (click here for instructions)
    • Do truth files need to be updated ("okified")?
      • after the reviewer has approved these changes, run okify_regtests to update the truth files
• if a JIRA ticket exists, make sure it is resolved properly

[emolter]

https://github.com/spacetelescope/RegressionTests/actions/runs/21761144518

[codecov]

Codecov Report

❌ Patch coverage is 64.70588% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.53%. Comparing base (bbf1e5f) to head (51e73a0).
⚠️ Report is 66 commits behind head on main.

Files with missing lines	Patch %	Lines
jwst/lib/exposure_types.py	42.85%	4 Missing ⚠️
jwst/datamodels/container.py	50.00%	1 Missing ⚠️
jwst/datamodels/library.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10237      +/-   ##
==========================================
+ Coverage   85.98%   86.53%   +0.55%     
==========================================
  Files         368      366       -2     
  Lines       38457    38455       -2     
==========================================
+ Hits        33066    33277     +211     
+ Misses       5391     5178     -213     

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