[Bug] llmagent is only compatible with functiontool where input is a struct

[yarolegovich]

Problem

functiontool can be used in contexts other than llmagent so any Type parameter constraints make sense:

type Func[TArgs, TResults any] func(tool.Context, TArgs) (TResults, error)

But llmagent is incompatible with functiontools where input is a non-struct type, because genai.FunctionCall.Args is a map:

Args map[string]any `json:"args,omitempty"`

The result is that an agent can never succeed in calling such tool.

Desired behavior

One of:

1. Validate tools in llmagent.New and report tool incompatibility as an error.
2. Make llmagent wrap such tools in a special decorator which will be handling unboxing of llm-provided arguments of form like map[string]any{"input": arg}. It will need to override input schema as well.

Reproduction

The agent will never succeed in running the super_tool:

func main() {
	ctx := context.Background()

	model, err := gemini.NewModel(ctx, "gemini-2.5-flash", &genai.ClientConfig{
		APIKey: os.Getenv("GOOGLE_API_KEY"),
	})
	if err != nil {
		log.Fatalf("Failed to create model: %v", err)
	}

	superTool, err := functiontool.New(functiontool.Config{Name: "super_tool"}, functiontool.Func[string, string](
		func(ctx tool.Context, input string) (string, error) {
			return "Hello", nil
		},
	))
	if err != nil {
		log.Fatalf("Failed to create a tool: %v", err)
	}

	a, err := llmagent.New(llmagent.Config{
		Name:        "super_tool_caller",
		Model:       model,
		Description: "Agent to invoke the super_tool",
		Instruction: "Your SOLE purpose is to invoke the super_tool and pass the result to me.",
		Tools:       []tool.Tool{superTool},
	})
	if err != nil {
		log.Fatalf("Failed to create agent: %v", err)
	}

	l := full.NewLauncher()
	if err = l.Execute(ctx, &launcher.Config{AgentLoader: agent.NewSingleLoader(a)}, os.Args[1:]); err != nil {
		log.Fatalf("Run failed: %v\n\n%s", err, l.CommandLineSyntax())
	}
}