assistant2: Push logic for adding thread context down into the ContextStore (#22855)

This PR takes the logic for adding thread context out of the
`ThreadContextPicker` and pushes it down into the `ContextStore`.

Release Notes:

- N/A

crates/assistant2/src/context_store.rs

@@ -4,7 +4,7 @@ use std::sync::Arc;
 
 use anyhow::{anyhow, bail, Result};
 use collections::{HashMap, HashSet};
-use gpui::{ModelContext, SharedString, Task, WeakView};
+use gpui::{Model, ModelContext, SharedString, Task, WeakView};
 use language::Buffer;
 use project::{ProjectPath, Worktree};
 use workspace::Workspace;
@@ -229,6 +229,14 @@ impl ContextStore {
         });
     }
 
+    pub fn add_thread(&mut self, thread: Model<Thread>, cx: &mut ModelContext<Self>) {
+        if let Some(context_id) = self.included_thread(&thread.read(cx).id()) {
+            self.remove_context(&context_id);
+        } else {
+            self.insert_thread(thread.read(cx));
+        }
+    }
+
     pub fn insert_thread(&mut self, thread: &Thread) {
         let context_id = self.next_context_id.post_inc();
         self.threads.insert(thread.id().clone(), context_id);