Fix disposable leak warning from in-process node.js file watcher (#320246)

NodeJSFileWatcherLibrary.watch() awaited doWatch() and then called
this._register() on the result. If the watcher was disposed during that
async window, _store was already disposed, producing a "disposable added
to a disposed store" warning and leaking the fs.watch handle.

Use thenRegisterOrDispose so the resolved handle is disposed instead of
registered when the watcher is torn down mid-flight. This is common in
the agent host where non-recursive watching runs in-process and
SessionCustomizationDiscovery churns watchers on file changes.

Fixes #320245

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

Author: roblourens

src/vs/platform/files/node/watcher/nodejs/nodejsWatcherLib.ts

@@ -112,7 +112,7 @@ export class NodeJSFileWatcherLibrary extends Disposable {
 				return;
 			}
 
-			this._register(await this.doWatch(stat.isDirectory()));
+			await thenRegisterOrDispose(this.doWatch(stat.isDirectory()), this._store);
 		} catch (error) {
 			if (error.code !== 'ENOENT') {
 				this.error(error);