Last follow-up of https://github.com/zed-industries/zed/pull/3002
Fixes
https://zed-industries.slack.com/archives/C04S6T1T7TQ/p1695281196667609

Copilot sends multiple events about its LSP server readiness, not
necessarily recreating the server from scratch (e.g. due to re-sign in
action). Avoid re-adding same log subscriptions on the same LSP server,
which causes panics.

Release Notes:

- N/A