Collapse long user messages

[vipmax]

Motivation

• User messages with large logs could flood the conversation UI and make it hard to navigate, so long user messages should be collapsed by default.

Description

• Add collapse/expand behavior to user message bodies in anycode/components/agent/AcpMessage.tsx using a ref that detects overflow and a local isExpanded state to toggle view.
• Add an accessible Show more / Show less button with aria-expanded and a CSS-limited collapsed height in anycode/components/agent/AcpMessage.css (max-height: 180px).
• Ensure attachments and existing actions (Undo) continue to render correctly when messages are collapsed.
• Add an E2E test in anycode/tests/demo.spec.ts to verify the collapsed default state and the expand/collapse interaction.

Testing

• Ran pnpm build:demo which completed successfully (build warnings noted but build finished).
• Ran the targeted Playwright test with pnpm exec playwright test --grep "collapse and expand long user messages" and the test passed (1 passed).
• Verified git diff --check and committed changes as Collapse long user messages.

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Codex Task