MM-30865 Invalid Participants handling on post delete in thread (#16353)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/sqlstore/post_store.go

@@ -1937,7 +1937,6 @@ func (s *SqlPostStore) cleanupThreads(postId, rootId, userId string, permanent b
 		}
 		if thread != nil {
 			thread.ReplyCount -= 1
-			thread.Participants = thread.Participants.Remove(userId)
 			if _, err = s.Thread().Update(thread); err != nil {
 				return errors.Wrap(err, "failed to update thread")
 			}

store/storetest/thread_store.go

@@ -4,12 +4,13 @@
 package storetest
 
 import (
+	"testing"
+	"time"
+
 	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/mattermost/mattermost-server/v5/store"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
-	"testing"
-	"time"
 )
 
 func TestThreadStore(t *testing.T, ss store.Store, s SqlSupplier) {
@@ -120,7 +121,7 @@ func testThreadStorePopulation(t *testing.T, ss store.Store) {
 		require.Nil(t, err, "couldn't get thread")
 		require.NotNil(t, thread)
 		require.Equal(t, int64(1), thread.ReplyCount)
-		require.ElementsMatch(t, model.StringArray{newPosts[0].UserId}, thread.Participants)
+		require.ElementsMatch(t, model.StringArray{newPosts[0].UserId, newPosts[1].UserId}, thread.Participants)
 	})
 
 	t.Run("Update reply should update the UpdateAt of the thread", func(t *testing.T) {
@@ -197,7 +198,7 @@ func testThreadStorePopulation(t *testing.T, ss store.Store) {
 		thread2, err := ss.Thread().Get(rootPost.RootId)
 		require.Nil(t, err)
 		require.EqualValues(t, thread2.ReplyCount, 1)
-		require.Len(t, thread2.Participants, 1)
+		require.Len(t, thread2.Participants, 2)
 	})
 
 	t.Run("Deleting root post should delete the thread", func(t *testing.T) {