Expose public Go APIs for model test functionality

description

The CLI currently implements functionality that is useful outside a command-line process, but the reusable pieces live under internal and are not consumable by external Go applications.

We would like to programmatically use the equivalent of fga model test, given an in-memory struct matching the documented .fga.yaml test file format, execute the tests and return structured results/summary.

example

func main() {

	result, err := modeltest.Run(ctx, modeltest.StoreData{
		Model: "...",
		Tuples: []client.ClientContextualTupleKey{...},
		Tests: []modeltest.ModelTest{...},
	}, modeltest.WithMaxTypesPerAuthorizationModel(100))

  	if result.Summary.Passing {
		// ...
  	}
}

Start by tracing the existing CLI implementation of fga model test and the documented .fga.yaml test format. Define the public modeltest entry point around the shown StoreData, ModelTest, options, and structured summary; done means an external Go application can run equivalent tests and inspect the result.