"Keep addressed agents active" should insert a space after the agent mentions

**Describe the bug**

When "Keep addressed agents active" is enabled, a mention for the agent(s) is inserted into the chat input, but no space is inserted after, so if the user starts typing the mention will be undone and converted to part of the user's input instead of the intended mention. The user will have to manually hit spacebar to avoid this which undermines the convenience of this feature

**Steps to reproduce**
1. In Settings -> Agents, enable `Keep addressed agents active`
2. Message an agent
3. Follow up with the agent

**Expected behavior**
What did you expect to happen?

**Version and platform**
Find your version at the bottom of the Settings sidebar. Write "unknown" if you can't determine it.
- Buzz version: 0.5.5
- OS: macOS 26.6

**Logs / additional context**