fix: backspace when StyleSourceInput is empty

[aleclarson]

Description

At some point, the local token was moved to the start of the style source array, but the backspace logic was never updated. This led to the 2nd-to-last source being detached, which is unexpected.

Steps for reproduction

1. Select a layer with 2+ style sources
2. Press the Command+Enter keys to focus the style sources input.
3. Press the Backspace key and notice the 2nd-to-last source gets detached, but it should be the last source getting detached

Code Review

• hi @kof, I need you to do
  • detailed review (read every line)
  • test it on preview

Before requesting a review

• made a self-review
• added inline comments where things may be not obvious (the "why", not "what")

Before merging

• tested locally and on preview environment (preview dev login: 0000)
• updated test cases document
• added tests

[TrySound]

I think current behavior makes sense

1. local style source cannot be deleted
2. caret is placed between tokens and local style source so it is expected backspace should remove things on the left

With your change caret removes a thing on the right and then tokens cannot be removed at all

Screen.Recording.2025-03-28.at.14.21.20.mov

At some point, the local token was moved to the start of the style source array,

This should not be possible. We enforce local styles to be placed in the end. If placed at the start it must be a bug.

[kof]

I agree with @TrySound, should we close?