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/eval/Cargo.toml

@@ -30,7 +30,6 @@ chrono.workspace = true
 clap.workspace = true
 client.workspace = true
 collections.workspace = true
-context_server.workspace = true
 dirs.workspace = true
 dotenv.workspace = true
 env_logger.workspace = true

crates/eval/src/eval.rs

@@ -426,7 +426,6 @@ pub fn init(cx: &mut App) -> Arc<AgentAppState> {
     language_model::init(client.clone(), cx);
     language_models::init(user_store.clone(), client.clone(), fs.clone(), cx);
     languages::init(languages.clone(), node_runtime.clone(), cx);
-    context_server::init(cx);
     prompt_store::init(cx);
     let stdout_is_a_pty = false;
     let prompt_builder = PromptBuilder::load(fs.clone(), stdout_is_a_pty, cx);