* return LanguageModelChatMessageRole and the simple chat message LanguageModelChatMessage2 because all types became same, same

* deprecate the other types and prepare deletion
* move System-role into its own proposal because of API and RAI concerns
* more API notes

src/vscode-dts/vscode.proposed.languageModelSystem.d.ts

@@ -0,0 +1,17 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+declare module 'vscode' {
+
+	// https://github.com/microsoft/vscode/issues/206265
+
+	// TODO@API don't have this dedicated type but as property, e.g anthropic doesn't have a system-role, see
+	// https://github.com/anthropics/anthropic-sdk-typescript/blob/c2da9604646ff103fbdbca016a9a9d49b03b387b/src/resources/messages.ts#L384
+	// So, we could have `LanguageModelChatRequestOptions#system` which would be more limiting but also more natural?
+
+	export enum LanguageModelChatMessageRole {
+		System = 3
+	}
+}