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

### Is there an existing issue for this?

- [x] 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 on request or response models.
• Response bodies typed TEnum?[].

The similar query-parameter scenario, TEnum?[] nullableValues, was addressed separately in PR #[68250](https://github.com/dotnet/aspnetcore/pull/68250).

### Expected Behavior

Nullable collection elements should keep their nullability:
```
"items": {
"oneOf": [
{ "type": "null" },
{ "$ref": "#/components/schemas/TestEnum" }
]
}
```

### Steps To Reproduce

_No response_

### Exceptions (if any)

_No response_

### .NET Version

_No response_

### Anything else?

_No response_