gpui: Fix follow_tail not re-engaging after scrollbar drag

[timvermeulen]

Fixes an inconsistency with the way scrolling interacts with the follow_tail GPUI list feature:

• Scrolling normally sets follow_state to FollowState::Tail { is_following: false }, so later scrolling to the bottom re-engages follow_tail
• Scrolling by dragging the scrollbar previously set follow_state to FollowState::Normal, preventing follow_tail from re-engaging after scrolling to the bottom

As far as I can tell this behavior was unintentional, despite there being a test that exercised this exact behavior that follow_tail doesn't re-engage after dragging the scrollbar — my best guess is that that test was derived from the bugged scrollbar handler logic, and not based on any actual intent for scrollbar drags to prevent follow_tail from re-engaging.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Fixed agent panel auto-scroll not re-engaging after interacting with the scrollbar