Fix an issue with updateReadonly perf

[pwang347]

Fixes #280253
If there's a large amount of customizations, we don't want to fire readonly change events in rapid succession. Let's debounce / group these.

[github-actions]

Screenshot Changes

Base: 34754a86 Current: 8e99269e

Changed (2)

chat/aiCustomizations/aiCustomizationManagementEditor/McpBrowseMode/Light

Before	After

editor/inlineCompletions/other/JumpToHint/Dark

Before	After

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blocks-ci screenshots changed

Replace the contents of test/componentFixtures/blocks-ci-screenshots.md with:

Updated blocks-ci-screenshots.md

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#### editor/codeEditor/CodeEditor/Dark
![screenshot](https://hediet-screenshots.azurewebsites.net/images/cb32a3e854b5734fe5aaca2318f2e0a42ee821b05ea97883ea42c5ba95edb3c3)

#### editor/codeEditor/CodeEditor/Light
![screenshot](https://hediet-screenshots.azurewebsites.net/images/42624fbba5e0db7f32c224b5eb9c5dd3b08245697ae2e7d2a88be0d7c287129b)

[Copilot]

Pull request overview

This PR addresses a performance issue caused by repeated updateReadonly calls firing onDidChangeReadonly for each file, which can cascade into expensive event handling (notably in prompt file registration).

Changes:

• Extend IFilesConfigurationService.updateReadonly to accept URI[] and batch the readonly override updates into a single onDidChangeReadonly event.
• Batch readonly updates in PromptsService (both provider listing and contributed file registration) to avoid per-file events.
• Add browser tests to validate event batching behavior for updateReadonly with single and multiple resources.

Show a summary per file

File	Description
src/vs/workbench/services/filesConfiguration/test/browser/filesConfigurationService.test.ts	Adds tests ensuring updateReadonly batches events for arrays and handles empty arrays/reset correctly.
src/vs/workbench/services/filesConfiguration/common/filesConfigurationService.ts	Updates the service contract and implementation to support URI[] and fire a single readonly-change event per batch.
src/vs/workbench/contrib/chat/common/promptSyntax/service/promptsServiceImpl.ts	Batches prompt-file readonly updates to prevent renderer freezes caused by per-file readonly change events.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/common/promptSyntax/service/promptsServiceImpl.ts:948

• The microtask flush calls filesConfigService.updateReadonly(...) without awaiting/handling the Promise. Please make the intent explicit (e.g. void ...) and add .catch(...) logging to avoid potential unhandled rejections (especially if a different IFilesConfigurationService implementation throws).

			queueMicrotask(() => {
				const uris = this._pendingReadonlyUris;
				this._pendingReadonlyUris = [];
				this._pendingReadonlyFlush = false;
				this.filesConfigService.updateReadonly(uris, true);
			});

• Files reviewed: 3/3 changed files
• Comments generated: 3