Chat interface includes backticks at end of selected code

[JasonWeill]

Description

When I send a message to Jupyternaut using the chat interface and I include the selection with my prompt, the code has three visible backticks at the end.

Reproduce

1. Open examples/code.ipynb and select some code in it.
2. Open the chat panel and ask Jupyternaut to explain code. Check the box to include the selection.
3. Observe that the outgoing prompt has a closing triple-backtick visible in the code (see screen shot above)

Expected behavior

The triple-backtick to end a code block is on a line by itself and is not visible in the chat panel.