Remove JsonSchema, use System.Text.Json.Schema instead

[Joy-less]

Support for JSON schemas was recently added by @mili-tan in #150.
What I wasn't aware of is that C# already contains a built-in way of generating JSON schemas, fully compatible with System.Text.Json and its various attributes:
https://devblogs.microsoft.com/dotnet/system-text-json-in-dotnet-9

Schemas generated this way are already compatible with OllamaSharp since format is of type object?.

This pull request removes the JsonSchema class (which doesn't contain support for enums, fields, etc) and encourages the user to use JsonSerializerOptions.Default.GetJsonSchemaAsNode instead.

Previously:

public class Dialog {
    public DialogLine[] DialogLines { get; set; } = [];
}
public class DialogLine {
    public string Name { get; set; }
    public string Speech { get; set; }
}
OllamaSharp.JsonSchema format = JsonSchema.ToJsonSchema(typeof(Dialog));

Now:

public class Dialog {
    public required DialogLine[] DialogLines { get; set; } = [];
}
public class DialogLine {
    public required string Name { get; set; }
    public required string Speech { get; set; }
}
System.Text.Json.Nodes.JsonNode format = JsonSerializerOptions.Default.GetJsonSchemaAsNode(typeof(Dialog));

System.Text.Json.Schema can be used in frameworks prior to .NET 9 by installing the latest official System.Text.Json NuGet package.

[awaescher]

I'd love to merge this, but would you please provide one or more unit tests to make sure everything works as expected?

[Joy-less]

Hi @awaescher, I'm not sure how this could be tested without calling the AI so I added a functional test which passed 5 times in a row:

One note is that the AI sometimes seemed to return null when I didn't instruct it to return valid JSON.

Another note is this API is added in .NET 9.0 so I needed to bump the test framework (or add a package, but we are already bumping the test framework in #162.)

[awaescher]

Looks good to me, awesome work once again, @Joy-less