feat(chat-sessions): show user upload attachments in session details

[squarezw]

Summary

• Session details and chat history now show the files a user uploaded on that turn.
• Depends on ragent-service#159, which persists attachment metadata on chat_session_detail.
• Turns recorded before that backend change have no stored attachments, so this block stays empty for them.

Test plan

• Merge / run ragent-service#159 first and apply chat_session_detail.attachments if the column is missing.
• Start a chat, attach a file, send a message.
• Open the session in 会话管理 → 详情 and confirm the file appears under the question; click to preview or download.
• Reload the same session in chat and confirm the attachment chip is back on the user message.
• Open an older session (no stored attachments) and confirm the details page still loads.

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