fix(schema): backport nullability round-trip handling to v2

[Copilot]

Pull Request

Description

Backports PR microsoft#2994 to preserve nullability when OpenAPI 3.1 schemas are downgraded to 3.0 and read back.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Related Issue(s)

Changes Made

• Serialization: emit enum: [null] and nullable: true for null-only 3.0 schemas; stop inferring parent types from nullable compositions.
• Deserialization: preserve JSON null enum entries and normalize null-only enums back to JsonSchemaType.Null.
• Coverage: add 3.0 round-trip tests and update affected schema expectations.

enum:
  - null
nullable: true

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• All existing tests pass

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Versions applicability

• My change applies to the version 1.X of the library, if so PR link:
• My change applies to the version 2.X of the library, if so PR link:
• My change applies to the version 3.X of the library, if so PR link:
• I have evaluated the applicability of my change against the other versions above.

See the contributing guidelines for more information about how patches are applied across multiple versions.

Additional Notes