Chat session attachment support

Updated the assets service to allow chat sessions to upload and read attachments via the /rpc/assets.uploadChatAttachment and /rpc/assets.serveChatAttachment endpoints.

This extends the attachment functionality introduced in v0.17.0 to work with session-based authentication, enabling seamless file sharing within chat experiences.

Features

• Session-based attachment access #1201  - Chat sessions can now upload and read attachments through the assets service endpoints. (Author: @disintegrator )

Bug fixes

• External OAuth authentication #1239  - Fixed an issue where the server would not return 401 for external OAuth servers when a gram-chat-session header was present. (Author: @walker-tx )