activePullRequest tool not attached on new chats

[eleanorjboyd]

Testing #253045

• checkout a PR
• chat mode correctly adds the activePullRequest implicitly
• create new chat (with plus)
• no longer has activePullRequest

[osortega]

I'm on the fence about this one, it might be a bit too much to constantly add the activePullRequest tool to every chat whenever you are in an active pull request. It might become a bit annoying to be constantly removing it if you don't need it as opposed to adding it through the tools menu if you need it.

That said, the checkout experience might change in the next iteration so I will move this to next milestone to make sure we keep this UX in mind.