Bump @jupyter/chat to 0.21.1

[brichet]

This PR bumps @jupyter/chat to 0.21.1.

It should not been merged before jupyterlab/jupyter-chat#397 get merged and released (it breaks the tool UI update

ai/src/chat-model.ts

Line 614 in 6fe7c0c

private _updateToolCallUI(

).

Also, this PR may require some changes on several other opened PRs:

• Use jupyter-chat-components for tool call components #276: there are changes of mime model handling in the message content
• Save and restore chat #303: updating message content now triggers a signal, that should be used to trigger a chat saving (if autosave)
• Add support for images and pdf files #306: notebook has now a mimetype like other files, so we could probably use it directly instead of looking for file extensions

[jtpio]

Thanks!