fix(message-editor): restore pasted text chip expansion

[adboio]

Problem

A recent message-editor change made repeat-paste the only way to undo long-text chip conversion, removing the existing click-to-expand interaction. Users still expect a “Pasted text” chip to expand when clicked.

Changes

• Restore click-to-expand while keeping repeat-paste undo.
• Retain pasted source text in editor-local state instead of reading files through the UI host boundary.
• Cover the chip interaction with a focused component test.

How did you test this?

• pnpm exec biome check --write --unsafe packages/ui/src/features/message-editor/tiptap/MentionChipNode.ts packages/ui/src/features/message-editor/tiptap/MentionChipView.tsx packages/ui/src/features/message-editor/tiptap/MentionChipView.test.tsx packages/ui/src/features/message-editor/tiptap/extensions.ts packages/ui/src/features/message-editor/tiptap/useTiptapEditor.ts
• pnpm --filter @posthog/ui exec vitest run src/features/message-editor/tiptap/MentionChipView.test.tsx
• pnpm --filter @posthog/ui typecheck

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 2c3a3b1.}