[Diagram Editor] Split operation improvements

[xiyuoh]

Slightly similar to #170, the Split operation is currently not very intuitive:

• With the default being Split-Keyed, there isn't any text showing up on the edges branching out of Split. Personally it took a while to realize that I had to click on the edges to enter the keys.
• The editor allows Split-Keyed and Split-Sequence to co-exist without errors or warnings.

Screencast.From.2026-03-16.14-57-33.mp4

Would suggest to change the type to Split-Sequence so that users know to modify index/keys via the edges, since it will display a default index of 0. Also to add a warning similar to what we did for Join when we have both keyed and sequence types branching out from the same node.