fix: preserve scroll-to-bottom when handling tool events

[linw1995]

Fix scroll-to-bottom behavior during tool event handling

When viewing messages at the bottom, tool events (e.g. tool approval, text edits) should keep the view scrolled to the bottom instead of staying in place.

Changes:

• Add is_at_bottom() helper method to detect bottom position
• Preserve bottom state in on_tool_event() handling
• Optimize ensure_visible() to avoid unnecessary scroll calculations

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 47.88%. Comparing base (c3970cb) to head (966f207).
⚠️ Report is 3 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/coco-tui/src/components/messages.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #166      +/-   ##
==========================================
+ Coverage   47.59%   47.88%   +0.29%     
==========================================
  Files         100      104       +4     
  Lines       27316    27698     +382     
==========================================
+ Hits        13000    13264     +264     
- Misses      14316    14434     +118     

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