Chat - add null checks for editingSession across await boundaries (#309129)

awaitStatsForSession reads model.editingSession across multiple await
points. If the model is disposed during one of these awaits, the
editingSession property is nulled and the next access crashes.

Re-check model.editingSession after each await and bail out with
undefined when the session has gone away.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: roblourens

src/vs/workbench/contrib/chat/common/chat.ts

@@ -44,7 +44,14 @@ export async function awaitStatsForSession(model: IChatModel): Promise<IChatSess
 	}
 
 	await chatEditingSessionIsReady(model.editingSession);
+	if (!model.editingSession) {
+		return undefined;
+	}
+
 	await Promise.all(model.editingSession.entries.get().map(entry => entry.getDiffInfo?.()));
+	if (!model.editingSession) {
+		return undefined;
+	}
 
 	const diffs = model.editingSession.entries.get();
 	const reduceResult = diffs.reduce((acc, diff) => {