context_store: Refactor state management (#29910)

Because we instantiated `ContextServerManager` both in `agent` and
`assistant-context-editor`, and these two entities track the running MCP
servers separately, we were effectively running every MCP server twice.

This PR moves the `ContextServerManager` into the project crate (now
called `ContextServerStore`). The store can be accessed via a project
instance. This ensures that we only instantiate one `ContextServerStore`
per project.

Also, this PR adds a bunch of tests to ensure that the
`ContextServerStore` behaves correctly (Previously there were none).

Closes #28714
Closes #29530

Release Notes:

- N/A

crates/project/src/git_store/git_traversal.rs

@@ -244,9 +244,8 @@ mod tests {
     use git::status::{FileStatus, StatusCode, TrackedSummary, UnmergedStatus, UnmergedStatusCode};
     use gpui::TestAppContext;
     use serde_json::json;
-    use settings::{Settings as _, SettingsStore};
+    use settings::SettingsStore;
     use util::path;
-    use worktree::WorktreeSettings;
 
     const CONFLICT: FileStatus = FileStatus::Unmerged(UnmergedStatus {
         first_head: UnmergedStatusCode::Updated,
@@ -682,7 +681,7 @@ mod tests {
         cx.update(|cx| {
             let settings_store = SettingsStore::test(cx);
             cx.set_global(settings_store);
-            WorktreeSettings::register(cx);
+            Project::init_settings(cx);
         });
     }