internal/mcp: add resource conformance test

This uncovered a problem in how we represent resource results:
contents is a slice, not a single value.

I tried to use a file resource, but the server->client ListRoots
call didn't work because synthetic responses aren't supported.
Left a TODO.

I enhanced the hello example so I that could use the MCP Inspector to
work with resources.

The falseSchema change fixes a potential bug where schemas are
re-used, which is disallowed.

Change-Id: I1f0198043cb6221c2a03f4f83aa0bafbc71167f4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/677136
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>

examples/hello/main.go

@@ -9,6 +9,7 @@ import (
 	"flag"
 	"fmt"
 	"net/http"
+	"net/url"
 	"os"
 
 	"github.com/modelcontextprotocol/go-sdk/mcp"
@@ -43,6 +44,14 @@ func main() {
 		mcp.Property("name", mcp.Description("the name to say hi to")),
 	)))
 	server.AddPrompts(mcp.NewPrompt("greet", "", PromptHi))
+	server.AddResources(&mcp.ServerResource{
+		Resource: &mcp.Resource{
+			Name:     "info",
+			MIMEType: "text/plain",
+			URI:      "embedded:info",
+		},
+		Handler: handleEmbeddedResource,
+	})
 
 	if *httpAddr != "" {
 		handler := mcp.NewSSEHandler(func(*http.Request) *mcp.Server {
@@ -56,3 +65,25 @@ func main() {
 		}
 	}
 }
+
+var embeddedResources = map[string]string{
+	"info": "This is the hello example server.",
+}
+
+func handleEmbeddedResource(_ context.Context, _ *mcp.ServerSession, params *mcp.ReadResourceParams) (*mcp.ReadResourceResult, error) {
+	u, err := url.Parse(params.URI)
+	if err != nil {
+		return nil, err
+	}
+	if u.Scheme != "embedded" {
+		return nil, fmt.Errorf("wrong scheme: %q", u.Scheme)
+	}
+	key := u.Opaque
+	text, ok := embeddedResources[key]
+	if !ok {
+		return nil, fmt.Errorf("no embedded resource named %q", key)
+	}
+	return &mcp.ReadResourceResult{
+		Contents: []*mcp.ResourceContents{mcp.NewTextResourceContents(params.URI, "text/plain", text)},
+	}, nil
+}