[Go] When using the dotprompt plugin with a `ModelName` config, it will panic

[dhiaayachi]

Describe the bug
When creating a prompt with a ModelName Config, the provided model name is not considered and it will panic. This is due to a bug when looking up the model in:

genkit/go/plugins/dotprompt/genkit.go

Lines 207 to 225 in 2940fd5

	model := p.Model
	if model == nil {
	modelName := p.ModelName
	if pr.Model != "" {
	modelName = pr.Model
	}
	if modelName == "" {
	return nil, errors.New("dotprompt execution: model not specified")
	}
	provider, name, found := strings.Cut(modelName, "/")
	if !found {
	return nil, errors.New("dotprompt model not in provider/name format")
	}
	model := ai.LookupModel(provider, name)
	if model == nil {
	return nil, fmt.Errorf("no model named %q for provider %q", name, provider)
	}
	}

The model is shadowed inside the if statement and result to the lookedup model going out of context after the if statement.

To Reproduce

prompt, _ := dotprompt.Define("menu", `Suggest an item for the menu of a {{ type }} themed restaurant`, dotprompt.Config{ModelName: "googleai/gemini-1.5-flash", InputSchema: r.Reflect(Input{}), OutputSchema: os, OutputFormat: ai.OutputFormatJSON})

resp, _ := prompt.Generate(ctx,
			&dotprompt.PromptRequest{
				Variables: input,
				Config:    &ai.GenerationCommonConfig{Temperature: 1},
			},
			nil,
		)

Expected behavior

When a model name is provided, either use the provided model name or return an error if the model lookup fails.

Runtime (please complete the following information):
Not relevant

** Go version
Not relevant

Additional context
The panic will happen in line:

genkit/go/plugins/dotprompt/genkit.go

Line 227 in 2940fd5

resp, err := model.Generate(ctx, genReq, cb)

This is similar to #847 but not quite the same issue.

[dhiaayachi]

I think the fix is pretty simple, just replacing

model := ai.LookupModel(provider, name)

with

model = ai.LookupModel(provider, name)

Please confirm and I can open a PR with the fix.

[yukinagae]

@dhiaayachi Hi, I was able to reproduce the error you mentioned locally. It seems you're right about both the cause and the solution. I think it's a good idea to create a PR for it.

It would also be better to write an additional test, similar to the one below, to ensure that passing a ModelName config works as expected: https://github.com/firebase/genkit/blob/main/go/plugins/dotprompt/genkit_test.go#L46