SSE: Add support for SSE handler options (#508)

mcp/sse: add support to provide for options for SSE transport

- Add SSEOptions struct to define SSE handler options.
- Update Signature of NewSSEHandler to accept SSEOptions [braking
change]
- Update unit test
- Update example test

Fixes https://github.com/modelcontextprotocol/go-sdk/issues/507,
https://github.com/modelcontextprotocol/go-sdk/issues/503

mcp/sse.go

@@ -43,12 +43,18 @@ import (
 // [2024-11-05 version]: https://modelcontextprotocol.io/specification/2024-11-05/basic/transports
 type SSEHandler struct {
 	getServer    func(request *http.Request) *Server
+	opts         SSEOptions
 	onConnection func(*ServerSession) // for testing; must not block
 
 	mu       sync.Mutex
 	sessions map[string]*SSEServerTransport
 }
 
+// SSEOptions specifies options for an [SSEHandler].
+// for now, it is empty, but may be extended in future.
+// https://github.com/modelcontextprotocol/go-sdk/issues/507
+type SSEOptions struct{}
+
 // NewSSEHandler returns a new [SSEHandler] that creates and manages MCP
 // sessions created via incoming HTTP requests.
 //
@@ -62,13 +68,17 @@ type SSEHandler struct {
 // The getServer function may return a distinct [Server] for each new
 // request, or reuse an existing server. If it returns nil, the handler
 // will return a 400 Bad Request.
-//
-// TODO(rfindley): add options.
-func NewSSEHandler(getServer func(request *http.Request) *Server) *SSEHandler {
-	return &SSEHandler{
+func NewSSEHandler(getServer func(request *http.Request) *Server, opts *SSEOptions) *SSEHandler {
+	s := &SSEHandler{
 		getServer: getServer,
 		sessions:  make(map[string]*SSEServerTransport),
 	}
+
+	if opts != nil {
+		s.opts = *opts
+	}
+
+	return s
 }
 
 // A SSEServerTransport is a logical SSE session created through a hanging GET