fix: preserve parent description when simplifying nullable anyOf/oneOf

[timonrieger]

Summary

This PR fixes a schema simplification edge case in ModelUtils.simplifyOneOfAnyOfWithOnlyOneNonNullSubSchema(...).

When anyOf/oneOf contains one non-null schema and one null schema, the logic collapses to the non-null child.
Before this change, parent-level description could be lost during collapse.
Now, parent description is preserved when the child has no description, while still preserving child description precedence if already set.

Changes

• Preserve parent-level description when simplifying nullable anyOf/oneOf to a single child schema.
• Keep existing nullable propagation behavior.
• Add unit tests to verify:
  • parent description is preserved for anyOf
  • parent description is preserved for oneOf
  • child description is not overwritten by parent description

Validation

• Added/updated unit tests in ModelUtilsTest.
• Verified expected behavior for nullable union simplification and description precedence.

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---

Summary by cubic

Fixes loss of parent description when simplifying nullable anyOf/oneOf unions to a single schema. The parent description is copied only if the child has none; nullable propagation remains unchanged.

• Bug Fixes
  • Updated ModelUtils.simplifyOneOfAnyOfWithOnlyOneNonNullSubSchema(...) to preserve parent description while keeping child description precedence.
  • Added tests for anyOf, oneOf, and child-overrides-parent cases in ModelUtilsTest.

^{Written for commit 751ab5c. Summary will update on new commits.}

[cubic-dev-ai]

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