[BUG][Dart] Object type is missing from list of object in OAS3.1

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?

Description

With OAS3.1 it's no longer possible to generate typed lists for API responses. A response like this:

	"200": {
		"content": {
			"application/json": {
				"schema": {
					"properties": {
						"result": {
							"items": {
								"$ref": "#/components/schemas/Thing"
							},
							"type": "array"
						}
					},
					"type": "object"
				}
			}
		},
	}

... used to generate a model with a field List<Thing> result;, but when changing the openapi option to 3.1.0 the type of the result field becomes Object? instead.

openapi-generator version

v7.4.0 and v7.5.0

OpenAPI declaration file content or url and steps to reproduce

https://github.com/traed/openapi-dart-bug

Start with the reproduction project at https://github.com/traed/openapi-dart-bug and generate the Dart client from the supplied OAS3.1 declaration. Compare the generated response model with OAS3.0 behavior, focusing on the result field. Done means an array of Thing references is generated as List rather than Object?.