codegen fails to produce anyOf/oneOf with a null type

### Description

The code generator skips `anyOf` or `oneOf` that have nullable listed:

```
warning: Schema "null" is not supported, reason: "schema type", skipping [context: foundIn=Operations.OperationName.Output.Ok.Body.JsonPayload.DataPayloadPayload (#/paths/path/to/resource/GET/responses/200/content/json/DataPayload)/my_type]
```

### Reproduction

```json
"my_type": {
"anyOf": [
{
"$ref": "#/components/schemas/MyType"
},
{
"type": "null"
}
]
}
```

```json
"my_type": {
"oneOf": [
{
"$ref": "#/components/schemas/MyType"
},
{
"type": "null"
}
]
}
```

### Package version(s)

### Expected behavior

ideally it should produce an optional. But an enum would also work.

### Environment

swift 6.2
Xcode 26.0 beta 6

### Additional information

The only workaround I've found is to do something like this, but the library our back end is using to produce the openapi spec does not seem to support something like this:

```
"my_type": {
"$ref": "#/components/schemas/MyType",
"type": ["object", "null"]
},
```

Start at the code generator's schema-type handling and reproduce the issue with the provided anyOf and oneOf OpenAPI snippets. Done means nullable schemas are represented in generated output as an optional or enum, without the reported unsupported-null warning.

Written by the indexing model from the issue text.