agent2: New thread from summary (#36578)

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <agus@zed.dev>
Co-authored-by: Cole Miller <cole@zed.dev>

crates/agent2/src/history_store.rs

@@ -111,6 +111,10 @@ impl HistoryStore {
         }
     }
 
+    pub fn thread_from_session_id(&self, session_id: &acp::SessionId) -> Option<&DbThreadMetadata> {
+        self.threads.iter().find(|thread| &thread.id == session_id)
+    }
+
     pub fn delete_thread(
         &mut self,
         id: acp::SessionId,