perf(common): implement copy-on-write behavior in formatters

[liangfung]

Summary

• Implements Copy-on-Write: Introduces helper functions mapOrOriginal, filterOrOriginal, and mapMessagePartsOrOriginal to replace in-place array/object mutations inside formatters.ui.
• Immutable State Updates: Refactors compactTask, writeRenderWidgetOutput, and appendCheckpoint to perform immutable updates on messages and parts arrays instead of using in-place methods like unshift or push.
• Preserves Reference Equality: Ensures that unmodified parts and messages conserve reference equality across formatters, avoiding unnecessary re-renders in the React-based UI of vscode-webui.

Perf

Before

After

Test plan

• Run unit tests to verify copy-on-write behaviors and that UI snapshots are not mutated:
  • bun turbo test --filter=@getpochi/common
  • bun turbo test --filter=@getpochi/livekit
  • bun turbo test --filter=@getpochi/vscode-webui
• Verify that types check successfully:
  • bun tsc
• Format and lint checks:
  • bun check

🤖 Generated with Pochi | Task

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.45946% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 38.39%. Comparing base (df49e9e) to head (b36f5df).

Files with missing lines	Patch %	Lines
...ebui/src/features/chat/lib/on-override-messages.ts	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1905      +/-   ##
==========================================
+ Coverage   38.31%   38.39%   +0.07%     
==========================================
  Files         772      772              
  Lines       82354    82447      +93     
  Branches     3951     4046      +95     
==========================================
+ Hits        31556    31656     +100     
+ Misses      50681    50674       -7     
  Partials      117      117              

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