Don't add listener for webview webview resource load

[mjbvz]

Fixes #312367

[Copilot]

Pull request overview

This PR addresses the listener leak warning reported for WebviewElement resource loading by avoiding per-resource cancellation listeners and ensuring active transferable streams are properly closed on disposal.

Changes:

• Track active ReadableStream controllers during resource streaming to avoid registering many token.onCancellationRequested listeners.
• Close and clear all active stream controllers when the webview element is disposed.
• Add early _disposed guards in loadResource to avoid doing work/sending responses after disposal.

Show a summary per file

File	Description
src/vs/workbench/contrib/webview/browser/webviewElement.ts	Removes per-stream cancellation listener pattern and adds centralized stream controller cleanup on webview disposal to prevent listener leak warnings.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0