internal/mcp: simplify prompt API

Remove NewServerPrompt.

Schema inference is irrelevant for prompts.
All arguments are strings.
So using a struct is not as valuable as with tools.
The only advantage is catching misspellings at compile time.
It's not worth it.

Change-Id: I45aaa5d55c8940cc9e32dc277bd543f79d932519
Reviewed-on: https://go-review.googlesource.com/c/tools/+/682797
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

examples/hello/main.go

@@ -32,11 +32,11 @@ func SayHi(ctx context.Context, ss *mcp.ServerSession, params *mcp.CallToolParam
 
 // TODO(jba): it should be OK for args to be a pointer, but this fails in
 // jsonschema. Needs investigation.
-func PromptHi(ctx context.Context, ss *mcp.ServerSession, args HiArgs, _ *mcp.GetPromptParams) (*mcp.GetPromptResult, error) {
+func PromptHi(ctx context.Context, ss *mcp.ServerSession, params *mcp.GetPromptParams) (*mcp.GetPromptResult, error) {
 	return &mcp.GetPromptResult{
 		Description: "Code review prompt",
 		Messages: []*mcp.PromptMessage{
-			{Role: "user", Content: mcp.NewTextContent("Say hi to " + args.Name)},
+			{Role: "user", Content: mcp.NewTextContent("Say hi to " + params.Arguments["name"])},
 		},
 	}, nil
 }
@@ -48,7 +48,10 @@ func main() {
 	server.AddTools(mcp.NewServerTool("greet", "say hi", SayHi, mcp.Input(
 		mcp.Property("name", mcp.Description("the name to say hi to")),
 	)))
-	server.AddPrompts(mcp.NewServerPrompt("greet", "", PromptHi))
+	server.AddPrompts(&mcp.ServerPrompt{
+		Prompt:  &mcp.Prompt{Name: "greet"},
+		Handler: PromptHi,
+	})
 	server.AddResources(&mcp.ServerResource{
 		Resource: &mcp.Resource{
 			Name:     "info",