Clear autocomplete input on selection accept

[dbkr]

Fixes https://github.com/element-hq/element-web/issues/27194

Checklist

• [ ] Tests written for new code (and old code if feasible).
• [ ] New or updated public/exported symbols have accurate TSDoc documentation.
• [ ] Linter and other CI checks pass.
• [ ] Sign-off given on the changes (see CONTRIBUTING.md).