mcp: add a test for streamable sampling during a tool call

Add a test that attempts (and fails) to reproduce the bug reported in
issue #285.

For #285

mcp/streamable_test.go

@@ -40,6 +40,25 @@ func TestStreamableTransports(t *testing.T) {
 			// 1. Create a server with a simple "greet" tool.
 			server := NewServer(testImpl, nil)
 			AddTool(server, &Tool{Name: "greet", Description: "say hi"}, sayHi)
+			AddTool(server, &Tool{Name: "sample"}, func(ctx context.Context, req *ServerRequest[*CallToolParams]) (*CallToolResult, error) {
+				// Test that we can make sampling requests during tool handling.
+				//
+				// Try this on both the request context and a background context, so
+				// that messages may be delivered on either the POST or GET connection.
+				for _, ctx := range map[string]context.Context{
+					"request context":    ctx,
+					"background context": context.Background(),
+				} {
+					res, err := req.Session.CreateMessage(ctx, &CreateMessageParams{})
+					if err != nil {
+						return nil, err
+					}
+					if g, w := res.Model, "aModel"; g != w {
+						return nil, fmt.Errorf("got %q, want %q", g, w)
+					}
+				}
+				return &CallToolResultFor[any]{}, nil
+			})
 
 			// 2. Start an httptest.Server with the StreamableHTTPHandler, wrapped in a
 			// cookie-checking middleware.
@@ -81,7 +100,11 @@ func TestStreamableTransports(t *testing.T) {
 				Endpoint:   httpServer.URL,
 				HTTPClient: httpClient,
 			}
-			client := NewClient(testImpl, nil)
+			client := NewClient(testImpl, &ClientOptions{
+				CreateMessageHandler: func(context.Context, *ClientRequest[*CreateMessageParams]) (*CreateMessageResult, error) {
+					return &CreateMessageResult{Model: "aModel", Content: &TextContent{}}, nil
+				},
+			})
 			session, err := client.Connect(ctx, transport, nil)
 			if err != nil {
 				t.Fatalf("client.Connect() failed: %v", err)
@@ -119,6 +142,19 @@ func TestStreamableTransports(t *testing.T) {
 			if diff := cmp.Diff(want, got); diff != "" {
 				t.Errorf("CallTool() returned unexpected content (-want +got):\n%s", diff)
 			}
+
+			// 6. Run the "sampling" tool and verify that the streamable server can
+			// call tools.
+			result, err := session.CallTool(ctx, &CallToolParams{
+				Name:      "sample",
+				Arguments: map[string]any{},
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if result.IsError {
+				t.Fatalf("tool failed: %s", result.Content[0].(*TextContent).Text)
+			}
 		})
 	}
 }