feat(web): add copy button to assistant messages

[nerdneilsfield]

Summary / 概述

This PR adds a copy button to assistant chat messages in the hub web UI.
When a message contains visible reply text, a copy button appears on hover, matching the existing user-message interaction.

这个 PR 为 hub 的 Web 聊天界面补充了 assistant 消息的复制按钮。
当消息包含可见回复正文时，悬停会显示 copy 按钮，交互方式与现有 user 消息保持一致。

Changes / 变更内容

• Add a hover-only copy button to assistant messages

• Reuse the existing clipboard hook and copied-state feedback

• Extract assistant copy text into a small helper

• Ignore tool cards, reasoning blocks, and CLI output when copying

• Add focused unit tests for assistant copy-text extraction

• 为 assistant 消息新增仅悬停显示的 copy 按钮

• 复用现有剪贴板 hook 和复制成功反馈

• 抽出 assistant 正文复制文本的辅助函数

• 复制时忽略 tool card、reasoning 和 CLI output

• 为 assistant 文本提取补充聚焦单测

Verification / 验证

• bun run test -- src/components/AssistantChat/messages/assistantCopyText.test.ts
• bun run typecheck

Notes / 说明

This PR only includes assistant-side copy support.
The existing user-message copy button remains unchanged.

这个 PR 只补充 assistant 侧的复制能力。
现有 user 消息复制按钮未改动。

[github-actions]

Findings

• No diff-attributable issues found in the modified lines after checking the assistant message rendering/copy path against adjacent web code.

Summary

• Review mode: initial
• No issues found in the latest diff. Residual risk: coverage here is helper-level only; I did not find a component-level web test for the assistant copy button interaction in repo/docs.

Testing

• Not run (automation): bun is not available in this runner, so bun run test -- web/src/components/AssistantChat/messages/assistantCopyText.test.ts and bun typecheck could not be executed.

HAPI Bot