Enable Side Chats for all sessions

### Short summary

Right now, side chats are only enabled for sessions that include a PR, but it would be useful to be able to use side chats for all sessions.

### Affected version or release

_No response_

### Installation context

_No response_

### What happened?

https://github.com/github/app/issues/2835 - the previous issue was closed, claiming that it was fixed in another version, but it is still not fixed and enabled.

### Steps to reproduce

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### Expected behavior

_No response_

### Additional context

_No response_

Start by reviewing the previous issue, github/app#2835, and locating the session and side-chat entry points in the desktop app. Trace where side chats are currently restricted to sessions with a PR, then define and test the behavior for sessions without one. Done means side chats are available in every session.

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