Make chat session item timing properties readonly

Clarifying that timing properties can't be updated individually on managed chat session items. If you want the change to apply, you need to re-set the whole property

src/vscode-dts/vscode.proposed.chatSessionsProvider.d.ts

@@ -102,7 +102,7 @@ declare module 'vscode' {
 		 *
 		 * This is also called on first load to get the initial set of items.
 		 */
-		refreshHandler: (token: CancellationToken) => Thenable<void>;
+		readonly refreshHandler: (token: CancellationToken) => Thenable<void>;
 
 		/**
 		 * Fired when an item's archived state changes.
@@ -204,33 +204,33 @@ declare module 'vscode' {
 			/**
 			 * Timestamp when the session was created in milliseconds elapsed since January 1, 1970 00:00:00 UTC.
 			 */
-			created: number;
+			readonly created: number;
 
 			/**
 			 * Timestamp when the most recent request started in milliseconds elapsed since January 1, 1970 00:00:00 UTC.
 			 *
 			 * Should be undefined if no requests have been made yet.
 			 */
-			lastRequestStarted?: number;
+			readonly lastRequestStarted?: number;
 
 			/**
 			 * Timestamp when the most recent request completed in milliseconds elapsed since January 1, 1970 00:00:00 UTC.
 			 *
 			 * Should be undefined if the most recent request is still in progress or if no requests have been made yet.
 			 */
-			lastRequestEnded?: number;
+			readonly lastRequestEnded?: number;
 
 			/**
 			 * Session start timestamp in milliseconds elapsed since January 1, 1970 00:00:00 UTC.
 			 * @deprecated Use `created` and `lastRequestStarted` instead.
 			 */
-			startTime?: number;
+			readonly startTime?: number;
 
 			/**
 			 * Session end timestamp in milliseconds elapsed since January 1, 1970 00:00:00 UTC.
 			 * @deprecated Use `lastRequestEnded` instead.
 			 */
-			endTime?: number;
+			readonly endTime?: number;
 		};
 
 		/**