Improve context server lifecycle management (#20622)

This optimizes and fixes bugs in our logic for maintaining a set of
running context servers, based on the combination of the user's
`context_servers` settings and their installed extensions.

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/collab/Cargo.toml

@@ -78,6 +78,7 @@ uuid.workspace = true
 
 [dev-dependencies]
 assistant = { workspace = true, features = ["test-support"] }
+context_servers.workspace = true
 async-trait.workspace = true
 audio.workspace = true
 call = { workspace = true, features = ["test-support"] }