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/agent/src/active_thread.rs

@@ -3487,7 +3487,6 @@ fn open_editor_at_position(
 #[cfg(test)]
 mod tests {
     use assistant_tool::{ToolRegistry, ToolWorkingSet};
-    use context_server::ContextServerSettings;
     use editor::EditorSettings;
     use fs::FakeFs;
     use gpui::{TestAppContext, VisualTestContext};
@@ -3559,7 +3558,6 @@ mod tests {
             workspace::init_settings(cx);
             language_model::init_settings(cx);
             ThemeSettings::register(cx);
-            ContextServerSettings::register(cx);
             EditorSettings::register(cx);
             ToolRegistry::default_global(cx);
         });