Use proper streams for loading webview resources

[mjbvz]

Fixes #311836

Switches to stream content to webviews. Previously we always read the entire content into a buffer before sending it over. Now we use streams to transfer it as directly as possible. On Chrome and Firefox, transferable streams let us send the byte data directly from the renderer to the service worker

This speeds up loading of resources inside webview and reduces memory usage. It's most helpful for videos and other larger files

Markdown preview with many video files (cache disabled but service worker ready)

Before

After

[Copilot]

Pull request overview

This PR improves webview resource loading performance by switching from buffering entire local resources in memory to streaming them to the webview (including range support), which is particularly beneficial for large assets such as videos.

Changes:

• Add streaming-based resource delivery from host to service worker using chunk and end messages.
• Forward HTTP Range requests to the host so only requested bytes are read from disk and returned as 206 Partial Content.
• Bump service worker version and update preload wiring/CSP hash to support the new message protocol.

Show a summary per file

File	Description
src/vs/workbench/contrib/webview/electron-browser/webviewElement.ts	Removes now-unneeded stream-to-buffer logic specific to Electron webview element.
src/vs/workbench/contrib/webview/browser/webviewMessages.d.ts	Updates message contracts for streaming (range request + chunk/end events).
src/vs/workbench/contrib/webview/browser/webviewElement.ts	Sends streaming metadata + streams chunks via new message channels; updates SW version expectation.
src/vs/workbench/contrib/webview/browser/resourceLoading.ts	Adds range-aware readFileStream options and exposes total file size for headers.
src/vs/workbench/contrib/webview/browser/pre/service-worker.js	Implements streaming pipeline (TransformStream) and range forwarding; adjusts caching behavior.
src/vs/workbench/contrib/webview/browser/pre/index.html	Updates preload forwarding to support new channels and transfer chunk buffers.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/webview/browser/pre/service-worker.js:441

• The 416 response for an unrecognized Range header sets Content-Range: '*/*', which is not a valid Content-Range value for 416 responses (and it also omits Cross-Origin-Resource-Policy). Prefer a spec-compliant header value (e.g. bytes */<size> when known, or omit Content-Range if you can't provide it) and include the same access-control headers as other resource responses.

			return new Response(null, {
				status: 416,
				headers: {
					'Access-Control-Allow-Origin': '*',
					'Content-Range': '*/*',
				}
			});

• Files reviewed: 5/6 changed files
• Comments generated: 5