Floating transient docks don't open as windows

### Tested versions

4.6 7ed0b6167618f9c8c5df87c7e83da6003c081368

### System information

W10

### Issue description

#113024 is progressing smoothly and soon all docks will support floating layout (because there really isn't any reason not to enable it). There is an edge case that doesn't work properly - transient docks, i.e. docks whose tabs will appear and disappear based on context. As of right now, only TileMap/TileSet are affected.

Basically, when you make the dock floating and it gets closed, on next reappearance it will be docked in the last used slot, instead of being a window.

### Steps to reproduce

1. Add a TileMap
2. Right-click TileMap bottom tab and Make Floating
3. Deselect TileMap
4. Select it again
5. The dock is at the bottom again

### Minimal reproduction project (MRP)

N/A

Start by reproducing the behavior in the Godot editor with a TileMap or TileSet dock: make it floating, deselect it, and select it again. Trace the transient dock lifecycle and compare the stored floating state before and after it disappears. Done means the dock reappears as a window rather than returning to its previous docked slot.

Written by the indexing model from the issue text.