Custom JsonConverter causes OpenAPI request schema to be empty

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

In a .NET 9 (also happens in .NET 10 preview) web app (minimal API), when implementing a custom JsonConverter for a request model, the generated OpenAPI spec contains an empty object for the request body schema. If the converter is not registered, the request body schema is defined correctly.

### Expected Behavior

The OpenAPI spec should contain a correctly defined schema for the request model in cases where a custom JsonConverter is defined.

### Steps To Reproduce

Simple repro project repository can be found [here](https://github.com/helo-email/open-api-converter-issue):. Running this app will generate the following OpenAPI spec (note the empty schema for `CreateChannelRequest`):
```
{
"openapi": "3.0.1",
"info": {
"title": "open-api-test | v1",
"version": "1.0.0"
},
"servers": [
{
"url": "http://localhost:5161/"
}
],
"paths": {
"/channels": {
"post": {
"tags": [
"open-api-test"
],
"requestBody": {
"content": {
"application/json": {
"schema": {
"$ref": "#/components/schemas/CreateChannelRequest"
}
}
},
"required": true
},
"responses": {
"200": {
"description": "OK",
"content": {
"text/plain": {
"schema": {
"type": "string"
}
}
}
}
}
}
}
},
"components": {
"schemas": {
"CreateChannelRequest": { }
}
},
"tags": [
{
"name": "open-api-test"
}
]
}
```
Removing the JsonConverter registration results in the following (correct) schema for `CreateChannelRequest`:
```
{
"openapi": "3.0.1",
"info": {
"title": "open-api-test | v1",
"version": "1.0.0"
},
"servers": [
{
"url": "http://localhost:5161/"
}
],
"paths": {
"/channels": {
"post": {
"tags": [
"open-api-test"
],
"requestBody": {
"content": {
"application/json": {
"schema": {
"$ref": "#/components/schemas/CreateChannelRequest"
}
}
},
"required": true
},
"responses": {
"200": {
"description": "OK",
"content": {
"text/plain": {
"schema": {
"type": "string"
}
}
}
}
}
}
}
},
"components": {
"schemas": {
"CreateChannelRequest": {
"required": [
"name",
"enabled"
],
"type": "object",
"properties": {
"name": {
"type": "string"
},
"enabled": {
"type": "boolean"
}
}
}
}
},
"tags": [
{
"name": "open-api-test"
}
]
}
```

### Exceptions (if any)

_No response_

### .NET Version

9.0.7 (but also fails in 10.0.100-preview.6.25358.103)

### Anything else?

_No response_

Start with the linked simple repro project and run the minimal API with the custom JsonConverter registered and unregistered. Trace the OpenAPI schema generation for CreateChannelRequest and compare the two generated specifications. Done means the registered-converter case produces the same defined properties and required fields as the unregistered case, with regression coverage added in the relevant OpenAPI tests.

Written by the indexing model from the issue text.