MM-63923/MM-63924/MM-63925 Prevent deadlocks and constraint errors in UpdateSidebarCategories (#30965)

* MM-63925 Remove most nested transactions from channel_store_categories.go

There's one place which still has a nested transaction in
CreateInitialSidebarCategories, but that's because it's calling out to a
different part of the store. The only way to avoid that would be to
break the extraction like UpdateSidebarCategories does to update
preferences, but I chose not to follow that pattern here and leave it
as-is.

* MM-63923 Prevent deadlocks caused by updating multiple categories in a different order

* MM-63923 Prevent deadlocks while deleting from SidebarChannels

This could also have been resolved by sorting the categories, but
combining the queries seems a bit more elegant.

* MM-63924 Ensure adding SidebarChannels rows is idempotent

* Add additional test to cause deadlocks

* Prevent channels from appearing in a single category multiple times

* Other review feedback

server/channels/api4/channel_category.go

@@ -257,6 +257,8 @@ func validateSidebarCategories(c *Context, teamId, userId string, categories []*
 	return nil
 }
 
+// validateSidebarCategoryChannels returns a normalized slice of channel IDs by removing duplicates from it and
+// ensuring that it only contains IDs of channels in the given ChannelList.
 func validateSidebarCategoryChannels(c *Context, userId string, channelIds []string, channels model.ChannelList) []string {
 	var filtered []string
 
@@ -276,6 +278,8 @@ func validateSidebarCategoryChannels(c *Context, userId string, channelIds []str
 		}
 	}
 
+	filtered = model.RemoveDuplicateStringsNonSort(filtered)
+
 	return filtered
 }
 

server/channels/api4/channel_category_test.go

@@ -1063,7 +1063,7 @@ func TestValidateSidebarCategoryChannels(t *testing.T) {
 		require.Empty(t, filtered)
 	})
 
-	t.Run("should preserve duplicate channel IDs", func(t *testing.T) {
+	t.Run("should prevent duplicate channel IDs", func(t *testing.T) {
 		channels := model.ChannelList{
 			th.BasicChannel,
 		}
@@ -1075,8 +1075,8 @@ func TestValidateSidebarCategoryChannels(t *testing.T) {
 		}
 
 		filtered := validateSidebarCategoryChannels(c, th.BasicUser.Id, channelIds, channels)
-		require.Len(t, filtered, 2) // Function preserves duplicates as per implementation
-		require.Equal(t, []string{th.BasicChannel.Id, th.BasicChannel.Id}, filtered)
+		require.Len(t, filtered, 1)
+		require.Equal(t, []string{th.BasicChannel.Id}, filtered)
 	})
 }