assistant2: Add debug logging for initialization issues (#23722)

This PR adds some logging so we can debug the issues some folks have
been having with Assistant2 not getting initialized properly.

All the logs are prefixed with `[assistant2-debug]` so they're easier to
pick out of the logs, as well as find them later to clean up once we've
diagnosed the issue.

Release Notes:

- N/A

crates/assistant2/src/thread_store.rs

@@ -67,7 +67,9 @@ impl ThreadStore {
                 this
             })?;
 
+            log::info!("[assistant2-debug] reloading threads");
             this.update(&mut cx, |this, cx| this.reload(cx))?.await?;
+            log::info!("[assistant2-debug] finished reloading threads");
 
             Ok(this)
         })