internal/mcp/protocol: featureSet[T]

What the spec calls "lists" of features (tools, prompts and so on)
are actually sets. Define a common data structure we can use to
implement all of them, and use it for server tools and prompts.

Change-Id: I257d0776b87a746ff23cd49ebcc7b317f4ea65e1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/671359
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

mcp/mcp_test.go

@@ -131,25 +131,28 @@ func TestEndToEnd(t *testing.T) {
 	if err != nil {
 		t.Errorf("tools/list failed: %v", err)
 	}
-	wantTools := []protocol.Tool{{
-		Name:        "greet",
-		Description: "say hi",
-		InputSchema: &jsonschema.Schema{
-			Type:     "object",
-			Required: []string{"Name"},
-			Properties: map[string]*jsonschema.Schema{
-				"Name": {Type: "string"},
+	wantTools := []protocol.Tool{
+		{
+			Name:        "fail",
+			Description: "just fail",
+			InputSchema: &jsonschema.Schema{
+				Type:                 "object",
+				AdditionalProperties: falseSchema,
 			},
-			AdditionalProperties: falseSchema,
 		},
-	}, {
-		Name:        "fail",
-		Description: "just fail",
-		InputSchema: &jsonschema.Schema{
-			Type:                 "object",
-			AdditionalProperties: falseSchema,
+		{
+			Name:        "greet",
+			Description: "say hi",
+			InputSchema: &jsonschema.Schema{
+				Type:     "object",
+				Required: []string{"Name"},
+				Properties: map[string]*jsonschema.Schema{
+					"Name": {Type: "string"},
+				},
+				AdditionalProperties: falseSchema,
+			},
 		},
-	}}
+	}
 	if diff := cmp.Diff(wantTools, gotTools, cmpopts.IgnoreUnexported(jsonschema.Schema{})); diff != "" {
 		t.Fatalf("tools/list mismatch (-want +got):\n%s", diff)
 	}