feat(client): Add the NewStdioMCPClientWithIO function.

This function allows creating an `StdioMCPClient` using provided `io.Reader`
and `io.Writer`. This allows creating an MCP server and client in the same
process, which significanly simplifies testing.

Author: octo

client/stdio.go

@@ -34,6 +34,30 @@ type StdioMCPClient struct {
 	capabilities  mcp.ServerCapabilities
 }
 
+// NewStdioMCPClientWithIO returns a new stdio-based MCP client using existing
+// input, output, and logging streams instead of spawning a subprocess.
+// This is useful for testing and simulating client behavior.
+func NewStdioMCPClientWithIO(input io.Reader, output io.WriteCloser, logging io.ReadCloser) *StdioMCPClient {
+	client := &StdioMCPClient{
+		cmd:       nil,
+		stdin:     output,
+		stdout:    bufio.NewReader(input),
+		stderr:    logging,
+		responses: make(map[int64]chan RPCResponse),
+		done:      make(chan struct{}),
+	}
+
+	// Start reading responses in a goroutine and wait for it to be ready
+	ready := make(chan struct{})
+	go func() {
+		close(ready)
+		client.readResponses()
+	}()
+	<-ready
+
+	return client
+}
+
 // NewStdioMCPClient creates a new stdio-based MCP client that communicates with a subprocess.
 // It launches the specified command with given arguments and sets up stdin/stdout pipes for communication.
 // Returns an error if the subprocess cannot be started or the pipes cannot be created.
@@ -64,28 +88,11 @@ func NewStdioMCPClient(
 		return nil, fmt.Errorf("failed to create stderr pipe: %w", err)
 	}
 
-	client := &StdioMCPClient{
-		cmd:       cmd,
-		stdin:     stdin,
-		stderr:    stderr,
-		stdout:    bufio.NewReader(stdout),
-		responses: make(map[int64]chan RPCResponse),
-		done:      make(chan struct{}),
-	}
-
 	if err := cmd.Start(); err != nil {
 		return nil, fmt.Errorf("failed to start command: %w", err)
 	}
 
-	// Start reading responses in a goroutine and wait for it to be ready
-	ready := make(chan struct{})
-	go func() {
-		close(ready)
-		client.readResponses()
-	}()
-	<-ready
-
-	return client, nil
+	return NewStdioMCPClientWithIO(stdout, stdin, stderr), nil
 }
 
 // Close shuts down the stdio client, closing the stdin pipe and waiting for the subprocess to exit.
@@ -98,6 +105,11 @@ func (c *StdioMCPClient) Close() error {
 	if err := c.stderr.Close(); err != nil {
 		return fmt.Errorf("failed to close stderr: %w", err)
 	}
+
+	if c.cmd == nil {
+		return nil
+	}
+
 	return c.cmd.Wait()
 }