Fix missed renames in #22632 (#23688)

Fix a bug where a GPUI macro still used `ModelContext`
Rename `AsyncAppContext` -> `AsyncApp`
Rename update_model, read_model, insert_model, and reserve_model to
update_entity, read_entity, insert_entity, and reserve_entity

Release Notes:

- N/A

crates/assistant_slash_commands/src/diagnostics_command.rs

@@ -303,7 +303,7 @@ fn collect_diagnostics(
                 .await
                 .log_err()
             {
-                let snapshot = cx.read_model(&buffer, |buffer, _| buffer.snapshot())?;
+                let snapshot = cx.read_entity(&buffer, |buffer, _| buffer.snapshot())?;
                 collect_buffer_diagnostics(&mut output, &snapshot, options.include_warnings);
             }