feat(chat): support pasted text message parts

[liangfung]

Summary

• Implement end-to-end support for data-pasted-text message parts in vscode-webui, common, and livekit.
• Add convertPastedTextPartsForLLM format op to transform pasted texts into plain text before sending to the LLM without mutating the original message.
• Add components, hooks, tests, and stories for the pasted-text composer card and regression verification.

Screenshot

Test plan

• Run vitest unit tests in packages/common, packages/livekit, and packages/vscode-webui to verify correct formatting, auto-memory handling, and component behavior.
• Verify that pasted text parts are displayed properly as composer cards in the UI.
• Verify that model-visible text conversion works as expected and does not treat compact tags inside pasted text as compaction boundaries.

🤖 Generated with Pochi | Task

[codecov-commenter]

Codecov Report

❌ Patch coverage is 64.16465% with 148 lines in your changes missing coverage. Please review.
✅ Project coverage is 34.15%. Comparing base (5702baf) to head (28705fd).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...age/__stories__/pasted-text-regression.stories.tsx	0.00%	97 Missing and 1 partial ⚠️
...src/features/chat/components/create-task-input.tsx	0.00%	12 Missing ⚠️
...i/src/features/chat/components/chat-input-form.tsx	79.41%	7 Missing ⚠️
...es/vscode-webui/src/components/dev-mode-button.tsx	14.28%	6 Missing ⚠️
...code-webui/src/components/message/message-list.tsx	76.19%	5 Missing ⚠️
...ebui/src/features/chat/components/chat-toolbar.tsx	16.66%	5 Missing ⚠️
packages/vscode-webui/src/features/chat/page.tsx	0.00%	4 Missing ⚠️
...e-webui/src/features/chat/hooks/use-chat-status.ts	0.00%	3 Missing ⚠️
packages/common/src/base/formatters.ts	87.50%	2 Missing ⚠️
...e-webui/src/components/attachment-preview-list.tsx	0.00%	2 Missing ⚠️
... and 3 more

❌ Your patch status has failed because the patch coverage (64.16%) 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    #1929      +/-   ##
==========================================
+ Coverage   33.32%   34.15%   +0.83%     
==========================================
  Files         778      780       +2     
  Lines       83120    83465     +345     
  Branches     4890     4914      +24     
==========================================
+ Hits        27696    28507     +811     
+ Misses      55304    54837     -467     
- Partials      120      121       +1     

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