Add Agent Preview trait (#29760)

Like the title says

Release Notes:

- N/A

crates/component_preview/src/preview_support/active_thread.rs

@@ -0,0 +1,69 @@
+use languages::LanguageRegistry;
+use project::Project;
+use std::sync::Arc;
+
+use agent::{ActiveThread, ContextStore, MessageSegment, ThreadStore};
+use assistant_tool::ToolWorkingSet;
+use gpui::{AppContext, AsyncApp, Entity, Task, WeakEntity};
+use prompt_store::PromptBuilder;
+use ui::{App, Window};
+use workspace::Workspace;
+
+pub async fn load_preview_thread_store(
+    workspace: WeakEntity<Workspace>,
+    project: Entity<Project>,
+    cx: &mut AsyncApp,
+) -> Task<anyhow::Result<Entity<ThreadStore>>> {
+    cx.spawn(async move |cx| {
+        workspace
+            .update(cx, |_, cx| {
+                ThreadStore::load(
+                    project.clone(),
+                    cx.new(|_| ToolWorkingSet::default()),
+                    None,
+                    Arc::new(PromptBuilder::new(None).unwrap()),
+                    cx,
+                )
+            })?
+            .await
+    })
+}
+
+pub fn static_active_thread(
+    workspace: WeakEntity<Workspace>,
+    project: Entity<Project>,
+    language_registry: Arc<LanguageRegistry>,
+    thread_store: Entity<ThreadStore>,
+    window: &mut Window,
+    cx: &mut App,
+) -> Entity<ActiveThread> {
+    let context_store =
+        cx.new(|_| ContextStore::new(project.downgrade(), Some(thread_store.downgrade())));
+
+    let thread = thread_store.update(cx, |thread_store, cx| thread_store.create_thread(cx));
+    thread.update(cx, |thread, cx| {
+        thread.insert_assistant_message(vec![
+            MessageSegment::Text("I'll help you fix the lifetime error in your `cx.spawn` call. When working with async operations in GPUI, there are specific patterns to follow for proper lifetime management.".to_string()),
+            MessageSegment::Text("\n\nLet's look at what's happening in your code:".to_string()),
+            MessageSegment::Text("\n\n---\n\nLet's check the current state of the active_thread.rs file to understand what might have changed:".to_string()),
+            MessageSegment::Text("\n\n---\n\nLooking at the implementation of `load_preview_thread_store` and understanding GPUI's async patterns, here's the issue:".to_string()),
+            MessageSegment::Text("\n\n1. `load_preview_thread_store` returns a `Task<anyhow::Result<Entity<ThreadStore>>>`, which means it's already a task".to_string()),
+            MessageSegment::Text("\n2. When you call this function inside another `spawn` call, you're nesting tasks incorrectly".to_string()),
+            MessageSegment::Text("\n3. The `this` parameter you're trying to use in your closure has the wrong context".to_string()),
+            MessageSegment::Text("\n\nHere's the correct way to implement this:".to_string()),
+            MessageSegment::Text("\n\n---\n\nThe problem is in how you're setting up the async closure and trying to reference variables like `window` and `language_registry` that aren't accessible in that scope.".to_string()),
+            MessageSegment::Text("\n\nHere's how to fix it:".to_string()),
+        ], cx);
+    });
+    cx.new(|cx| {
+        ActiveThread::new(
+            thread,
+            thread_store,
+            context_store,
+            language_registry,
+            workspace.clone(),
+            window,
+            cx,
+        )
+    })
+}