Fix copilot state corruption when there are multiple buffers with the same remote id in different projects (#2569)

Fixes
https://linear.app/zed-industries/issue/Z-1511/thread-main-panicked-at-assertion-failed-left-==-right-left-local-0-1

Previously, when exchanging messages about buffers with a copilot
language server, we identified buffers using their **remote id**. This
caused problems when there were multiple projects open, where one or
more were remote, because buffers' remote ids are only unique within a
given project.

When you have multiple projects open, and one or more of the projects is
remote, it's pretty easy to have two buffers open with the same remote
id. In my testing, when this happened, copilot would stop working in
both buffers. But I believe that depending on the editing patterns that
occur in the two buffers, it could cause the crash reported in the
Linear issue above.

This PR changes our copilot logic to use buffers' local handle ids for
identifying them. This fixed the problems I was able to reproduce when
using copilot in both remote and local projects.

Release Notes:

- Fixed a crash that would sometimes occur when editing buffers after
having collaborated on a remote project.