Replace discriminated unions with simple unions in models

[candleindark]

Summary

Removes the use of discriminated unions in favor of plain Union[...] for the wasAssociatedWith, contributor, and about fields.

This has no effect on validation correctness: each union member has a distinct schemaKey: Literal[...] value, so Pydantic's smart union mode resolves the correct type without a discriminator.

The primary motivation is to enable translation to LinkML via pydantic2linkml, since LinkML has no comparable feature to discriminated unions (see dandi/pydantic2linkml#39).

Notes:
Use of discriminated unions was introduced to improve validation error messages per #244.

Test plan

• All existing tests pass (240 passed, 3 skipped)
• Analyze the effect of this change on meditor

🤖 Generated with Claude Code

[yarikoptic]

I would like to see the effect on schema.

DONE -- the diff: see/show the diff on jsonschema! I am surprised that our workflow didn't run at all -- we might want it to still run to visualize the diff, and then do its decision making on either to fail or not.... I will add release label just for that for now. We might want just to make it still run although not do categories

TODO: add to PR description analysis of why we had them to start with and analysis on possible effects on meditor

[candleindark]

Note: use of discriminated unions was introduced to improve validation error messages per #244.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.31%. Comparing base (9c694b5) to head (ce42f55).

Files with missing lines	Patch %	Lines
dandischema/models.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #385   +/-   ##
=======================================
  Coverage   48.31%   48.31%           
=======================================
  Files          19       19           
  Lines        2434     2434           
=======================================
  Hits         1176     1176           
  Misses       1258     1258           

Flag	Coverage Δ
unittests	48.31% <0.00%> (ø)

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