Previously we would still show a completion menu even when the user
typed an unrecognised mode with an argument,
e.g. `@something word`.
This PR ensures that we only show the completion menu, when the part
after the `@` is a known mode (e.g. `file`/`symbol`/`rule`/...)
Release Notes:
- Fix an issue where completions for `@mentions` in the agent panel
would sometimes not be dismissed when typing a space
Bennet Bo Fenner
630340d659