Changed files list defaults differently for local vs background sessions

[bamurtaugh]

Ref https://github.com/microsoft/vscode-internalbacklog/issues/6313

In my testing, it seems the changed files list defaults to expanded for background or cloud sessions, but minimized for local agent mode. I think I'd expect them both to appear the same (either both expanded or both minimized by default).

As I've been accustomed to reviewing the changed files list for background CLI sessions, I was confused when I was in a local agent mode chat editor and the list wasn't expanded:

[DonJayamanne]

Actually local and background files are always collapsed.
For cloud the file list is always expanded. i think we should change the default in cloud.

I.e. local has been around for a while and users are accustomed to it being collapsed, hence my suggestion to be consistent with that (existing) approach. @bamurtaugh /cc

Also the cloud version uses a separate widget for this (@osortega @joshspicer /cc)

[bamurtaugh]

Defaulting the cloud list to being collapsed to be consistent with the other lists makes sense to me.

[joshspicer]

We can set the multifilediffpart (shown in cloud only) to collapse, but that requires new chat part API. I don't think it's worth doing as a candidate