OpenAPI: nullability is dropped for collection elements whose type is componentized #68322

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When a collection's element type is a nullable value type that gets componentized into #/components/schemas/*, the generated document loses the element's nullability. The items schema is emitted as a bare $ref to the non-nullable component:

"listOfNullableEnums": {
  "type": "array",
  "items": { "$ref": "#/components/schemas/TestEnum" }
}

Affected shapes include, at minimum:
• Properties typed List<TEnum?> on request or response models.
• Response bodies typed TEnum?[].

The similar query-parameter scenario, TEnum?[] nullableValues, was addressed separately in PR #68250.

Expected Behavior

Nullable collection elements should keep their nullability:

"items": {
  "oneOf": [
    { "type": "null" },
    { "$ref": "#/components/schemas/TestEnum" }
  ]
}

Steps To Reproduce

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Exceptions (if any)

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.NET Version

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Anything else?

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