fix: fix nil pointer dereference error in error handler

[alexmt]

PR fixes the nil pointer dereference error in error response handler. The snippet below reproduces the issue:

package main

import (
	"context"
	"log"
	"os/exec"

	mcp "github.com/metoro-io/mcp-golang"
	"github.com/metoro-io/mcp-golang/transport/stdio"
)

func main() {
	cmd := exec.Command("npx", "-y", "@executeautomation/playwright-mcp-server")
	stdin, err := cmd.StdinPipe()
	if err != nil {
		log.Fatalf("Failed to get stdin pipe: %v", err)
	}
	stdout, err := cmd.StdoutPipe()
	if err != nil {
		log.Fatalf("Failed to get stdout pipe: %v", err)
	}

	if err := cmd.Start(); err != nil {
		log.Fatalf("Failed to start server: %v", err)
	}
	defer cmd.Process.Kill()

	transport := stdio.NewStdioServerTransportWithIO(stdout, stdin)
	client := mcp.NewClient(transport)

	if _, err := client.Initialize(context.Background()); err != nil {
		log.Fatalf("Failed to initialize: %v", err)
	}

	// nil value for cursor causes the error
	res, err := client.ListTools(context.Background(), nil)
	if err != nil {
		log.Fatalf("Failed to list res: %v", err)
	}
	for _, tool := range res.Tools {
		log.Printf("Tool: %s", tool.Name)
	}
}

[Copilot]

Pull Request Overview

This PR addresses a nil pointer dereference error in the error response handler by updating how the request ID is assigned and adding a test to verify error handling.

• Updated the handleResponse function in protocol.go to alter the assignment of the request ID.
• Added a new test in protocol_test.go to simulate and validate error response handling.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
internal/protocol/protocol_test.go	Added a test to verify that error responses are handled correctly.
internal/protocol/protocol.go	Modified the request ID initialization within the error and success branches of handleResponse.

[Copilot]

In the error branch of handleResponse, 'id' is not initialized with response.Id, potentially leading to a zero value being used later. Consider assigning id = response.Id in both branches to ensure consistent behavior.

[alexmt]

response.Id causes nil pointer dereference error

[rainu]

I have tested this change in my application - it works fine for me 👍

BTW: This will fix the issue #96