Include session id in request payload of WebSocketMessageHasBeenPosted plugin hook (#25928)

server/channels/app/plugin_api_tests/manual.test_websocket_session/main.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package main
+
+import (
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/plugin"
+)
+
+type Plugin struct {
+	plugin.MattermostPlugin
+	sessionCh chan string
+}
+
+func (p *Plugin) MessageWillBePosted(_ *plugin.Context, _ *model.Post) (*model.Post, string) {
+	return nil, <-p.sessionCh
+}
+
+func (p *Plugin) WebSocketMessageHasBeenPosted(connID, userID string, req *model.WebSocketRequest) {
+	p.sessionCh <- req.Session.Id
+}
+
+func main() {
+	plugin.ClientMain(&Plugin{
+		sessionCh: make(chan string, 1),
+	})
+}