Use read-only access methods for read-only entity operations (#31479)

Another follow-up to #31254

Release Notes:

- N/A

crates/copilot/src/copilot.rs

@@ -232,7 +232,7 @@ impl RegisteredBuffer {
                         Some(buffer.snapshot.version.clone())
                     })
                     .ok()??;
-                let new_snapshot = buffer.update(cx, |buffer, _| buffer.snapshot()).ok()?;
+                let new_snapshot = buffer.read_with(cx, |buffer, _| buffer.snapshot()).ok()?;
 
                 let content_changes = cx
                     .background_spawn({