MM-42810: Using websocket broadcast hook for permalink preview (#28627)

We use the newly introduced websocket broadcast hook system
to implement permalink preview efficiently. This is essentially
a re-do of https://github.com/mattermost/mattermost/pull/23812
using the new system.

https://mattermost.atlassian.net/browse/MM-42810
```release-note
NONE
```

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/web_broadcast_hooks_test.go

@@ -208,3 +208,59 @@ func TestAddMentionsAndAddFollowersHooks(t *testing.T) {
 		assert.Equal(t, `["`+userID+`"]`, msg.Event().GetData()["mentions"])
 	})
 }
+
+func TestPermalinkBroadcastHook(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+
+	session, err := th.Server.Platform().CreateSession(th.Context, &model.Session{
+		UserId: th.BasicUser.Id,
+	})
+	require.NoError(t, err)
+
+	wc := &platform.WebConn{
+		Platform: th.Server.Platform(),
+		Suite:    th.App,
+		UserId:   session.UserId,
+	}
+	hook := &permalinkBroadcastHook{}
+
+	refPost := th.CreatePost(th.BasicChannel)
+
+	th.BasicPost.Metadata.Embeds = append(th.BasicPost.Metadata.Embeds, &model.PostEmbed{Type: model.PostEmbedPermalink, Data: &model.Permalink{
+		PreviewPost: model.NewPreviewPost(refPost, th.BasicTeam, th.BasicChannel),
+	}})
+	originalJSON, err := th.BasicPost.ToJSON()
+	require.NoError(t, err)
+
+	wsEvent := model.NewWebSocketEvent(model.WebsocketEventPosted, "", th.BasicPost.ChannelId, "", nil, "")
+	th.BasicPost.Metadata.Embeds[0].Data = nil
+	removedJSON, err := th.BasicPost.ToJSON()
+	require.NoError(t, err)
+
+	wsEvent.Add("post", removedJSON)
+	msg := platform.MakeHookedWebSocketEvent(wsEvent)
+
+	// User has permission.
+	err = hook.Process(msg, wc, map[string]any{
+		"preview_channel": th.BasicChannel,
+		"post_json":       originalJSON,
+	})
+	require.NoError(t, err)
+
+	gotJSON, ok := msg.Get("post").(string)
+	require.True(t, ok)
+	require.Equal(t, originalJSON, gotJSON)
+
+	msg = platform.MakeHookedWebSocketEvent(wsEvent)
+	// User does not exist, and thus won't have permission to the channel.
+	wc.UserId = "otheruser"
+	err = hook.Process(msg, wc, map[string]any{
+		"preview_channel": th.BasicChannel,
+		"post_json":       originalJSON,
+	})
+	require.NoError(t, err)
+	gotJSON, ok = msg.Get("post").(string)
+	require.True(t, ok)
+	require.Equal(t, removedJSON, gotJSON)
+}