Look up extensions in the new index when reporting extension events (#9879)

This PR fixes a bug that was causing extension telemetry events to not
be reported.

We need to look up the extensions in the new index, as the extensions to
load won't be found in the old index.

Release Notes:

- N/A

crates/extension/src/extension_store.rs

@@ -761,7 +761,7 @@ impl ExtensionStore {
 
         if let Some(telemetry) = &self.telemetry {
             for extension_id in &extensions_to_load {
-                if let Some(extension) = self.extension_index.extensions.get(extension_id) {
+                if let Some(extension) = new_index.extensions.get(extension_id) {
                     telemetry.report_extension_event(
                         extension_id.clone(),
                         extension.manifest.version.clone(),