Agent sessions diff decoration doesn't disappear after undoing some changes

[Tyriar]

Testing #290241

Repro:

1. In agent sessions window (maybe doesn't matter)
2. Ask to make some changes
3. Undo them, 🐛 the changes remain in the sessions sidebar

[bpasero]

Seems related to tracking diff info for a session, I would like to get some help from @lszomoru and @connor4312 on this.