assistant2: Combine history views into one (#25293)

This PR combines the two history views in Assistant2 into one.

<img width="1309" alt="Screenshot 2025-02-20 at 5 34 37 PM"
src="https://github.com/user-attachments/assets/fbb08542-58b5-4930-8a20-254234e335fa"
/>

<img width="1309" alt="Screenshot 2025-02-20 at 5 34 41 PM"
src="https://github.com/user-attachments/assets/1174849e-edad-4e02-8bf3-bb92aafba4f8"
/>


Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -7,6 +7,7 @@ mod context;
 mod context_picker;
 mod context_store;
 mod context_strip;
+mod history_store;
 mod inline_assistant;
 mod inline_prompt_editor;
 mod message_editor;
@@ -40,7 +41,6 @@ actions!(
         ToggleModelSelector,
         RemoveAllContext,
         OpenHistory,
-        OpenPromptEditorHistory,
         OpenConfiguration,
         RemoveSelectedThread,
         Chat,