Fix Erase Terrain button not activating Terrain brush

[JustJ01]

Closes #4177

Description:

A bug was reported where clicking Erase Terrain in the Terrain Sets dock cleared the current terrain selection but did not switch the active tool to the Terrain Brush. This left users painting with whatever tool was previously selected (Stamp Brush) and broke the expected workflow.

Before:

In the video , you can see when Erase Terrain button was pressed , the current tool remained Stamp Brush instead of swtiching to Terrain Brush.

Screen.Recording.2026-02-21.155735.mp4

After:

After the change, clicking Erase Terrain always activates the Terrain Brush , restoring the intended behaviour.

Screen.Recording.2026-02-21.160846.mp4

[bjorn]

I've just removed that button in #4320 (not merged yet, but it's mostly just waiting for a slight name change). However, I noticed this issue is not fixed yet. Maybe you could have a look at rebasing your change onto the erase-terrain-selection branch?

[JustJ01]

Thanks @bjorn for the review, I will look at the codebase of that branch