Command prefixes can clash

**Describe the bug**
When there are multiple connections in the same room, it is possible for them to share the same command prefix (`!gl`, `!gh`, etc). If a command with a shared prefix is run, it will only be processed against one of the connections. This prevents being able to run commands for some of the connections that share a command prefix.

**To Reproduce**
Steps to reproduce the behavior:
1. Connect two GitLab projects to a room, and do not customize the command prefix from its default of `!gl`
2. Try to run `!gl` commands in that room
3. Notice that only one of the projects accepts commands

**Expected behavior**
It should be forbidden for multiple connections to share the same command prefix. If adding a new connection with a default prefix clashes with one already in use within a room, either the prefix should be modified to be unique, or the action of adding a connection should require the user to set a custom, unique prefix for it.

**Screenshots**
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**Additional context**
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Trace the connection-creation flow and the room command-prefix dispatch described in the report. Reproduce the collision with two GitLab connections, then ensure adding a connection prevents or resolves duplicate prefixes and verify that commands reach the intended connection.

Written by the indexing model from the issue text.