Chat emoji autocorrecting autoincorrectly

When I type `:-D` at the end of a chat message, I expect to get either the characters I typed, or a matching emoji. Instead it gets corrected to `:diamonds:` and doesn't send the chat, giving me a little editing task I didn't ask for.

Start by locating the chat composer’s emoji autocorrection and send handling, using the reported sequence `:-D` as the reproduction case. Done means the input remains as typed or becomes a matching emoji, and pressing Return sends the chat without leaving `:diamonds:` in the editor.

Written by the indexing model from the issue text.