mcp: support stateless streamable sessions (#277)

Support stateless streamable sessions by adding a GetSessionID function
to StreamableHTTPOptions. If GetSessionID returns "", the session is
stateless, and no validation is performed. This is implemented by
providing the session a trivial initialization state.

To implement this, some parts of #232 (distributed sessions) are copied
over, since they add an API for creating an already-initialized session.

In total, the following new API is added:
- StreamableHTTPOptions.GetSessionID
- ServerSessionOptions (a new parameter to Server.Connect)
- ServerSessionState
- ClientSessionOptions (a new parameter to Client.Connect, for symmetry)

For #10

mcp/server_example_test.go

@@ -31,13 +31,13 @@ func ExampleServer() {
 	server := mcp.NewServer(&mcp.Implementation{Name: "greeter", Version: "v0.0.1"}, nil)
 	mcp.AddTool(server, &mcp.Tool{Name: "greet", Description: "say hi"}, SayHi)
 
-	serverSession, err := server.Connect(ctx, serverTransport)
+	serverSession, err := server.Connect(ctx, serverTransport, nil)
 	if err != nil {
 		log.Fatal(err)
 	}
 
 	client := mcp.NewClient(&mcp.Implementation{Name: "client"}, nil)
-	clientSession, err := client.Connect(ctx, clientTransport)
+	clientSession, err := client.Connect(ctx, clientTransport, nil)
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -62,11 +62,11 @@ func createSessions(ctx context.Context) (*mcp.ClientSession, *mcp.ServerSession
 	server := mcp.NewServer(testImpl, nil)
 	client := mcp.NewClient(testImpl, nil)
 	serverTransport, clientTransport := mcp.NewInMemoryTransports()
-	serverSession, err := server.Connect(ctx, serverTransport)
+	serverSession, err := server.Connect(ctx, serverTransport, nil)
 	if err != nil {
 		log.Fatal(err)
 	}
-	clientSession, err := client.Connect(ctx, clientTransport)
+	clientSession, err := client.Connect(ctx, clientTransport, nil)
 	if err != nil {
 		log.Fatal(err)
 	}