aiCustomization: make plugin/extension/builtin files read-only

[connor4312]

aiCustomization: make plugin/extension/builtin files read-only

• Open plugin, extension, and built-in customization files as read-only
  in the editor, with a message explaining they cannot be edited.
• Extract isReadonlyStorage() helper to DRY the repeated storage type
  check used in both the open and delete actions.

Fixes #301292

(Commit message generated by Copilot)

[Copilot]

Pull request overview

Updates the AI Customization management actions so that customization files coming from non-user-editable sources (plugins/extensions/built-in) open as read-only, aligning the editor experience with the intended immutability of those artifacts.

Changes:

• Add isReadonlyStorage() helper to centralize the “extension/plugin/built-in” storage check.
• Attempt to open read-only storages in the editor as read-only with a dedicated read-only message.
• Reuse the same read-only storage check for the delete action gate.

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationManagement.contribution.ts:258

• This info dialog is now shown for plugin and built-in files as well (via isReadonlyStorage(storage)), but the title/detail still specifically mention extensions and recommend disabling the extension. Please update the localized strings (or branch on storage) so the guidance is correct for plugin and built-in sources too.

		if (isReadonlyStorage(storage)) {
			await dialogService.info(
				localize('cannotDeleteExtension', "Cannot Delete Extension File"),
				localize('cannotDeleteExtensionDetail', "Files provided by extensions cannot be deleted. You can disable the extension if you no longer want to use this customization.")
			);

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[joshspicer]

'built-in' for the sessions app should be editable (to let the user customize)