StreamableHTTPServerTransport should support multiple initialization

For stateless mode, mcp server should be able to handle multiple client request. Such as below usage

const server = new McpServer(
    {
      name: "mcp-typescript server on vercel",
      version: "0.1.0",
    },
    serverOptions
 );
 const statelessTransport = new StreamableHTTPServerTransport({
    sessionIdGenerator: ()=> undifined,
 });
server.connect(statelessTransport);
app.post('/mcp', (req, res) => {
  // only one  statelessTransport handle multi request
  // this is important for serverless mode
  statelessTransport.handleRequest(req, res, req.body);
});

Motivation and Context

How Has This Been Tested?

Unit test and local test

Breaking Changes

no

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context