chore(vscode-webui): stop passing full message array to tool parts

[liangfung]

Summary

Every tool part received the whole formatted renderMessages array, whose identity changes on each streaming tick, defeating memoization for all tool parts. Only attemptCompletion and renderWidget consumed it, and both only needed to know their position, so replace it with two booleans derived once per part: the existing isLastPart and a new isInLastAssistantMessage.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 62.50000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.31%. Comparing base (f2af902) to head (d74044e).

Files with missing lines	Patch %	Lines
...code-webui/src/features/tools/components/index.tsx	0.00%	5 Missing ⚠️
...s/tools/__stories__/attempt-completion.stories.tsx	0.00%	4 Missing ⚠️

❌ Your patch status has failed because the patch coverage (62.50%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1903      +/-   ##
==========================================
+ Coverage   38.30%   38.31%   +0.01%     
==========================================
  Files         772      772              
  Lines       82410    82354      -56     
  Branches     3951     4015      +64     
==========================================
- Hits        31566    31556      -10     
+ Misses      50727    50681      -46     
  Partials      117      117              

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