[gen] Missing import for zio.schema._ in generated code when response content has application/json with type: object.

[wi101]

Describe the bug
When generating Scala code from an OpenAPI JSON file containing a response with the following structure:

"content": {
  "application/json": {
    "schema": {
      "type": "object"
    }
  }
}

The generated code produces the following:

case class ResponseBody()
object ResponseBody {
  implicit val codec: Schema[ResponseBody] = DeriveSchema.gen[ResponseBody]
}

However, the required import import zio.schema._ is missing, causing compilation errors.

To Reproduce
Steps to reproduce the behaviour:

1. Use an OpenAPI JSON file with a response schema like the one above.
2. Run the code generation.
3. Observe the missing import zio.schema._ in the generated code.

Expected behaviour
The generated code should include the necessary import for zio.schema._ to ensure it compiles successfully.

Used version
zio-http version: 3.0.1