mcp: support system message is sampling (microsoft#250900)

Ref https://github.com/microsoft/vscode/issues/244162\#issuecomment-2946034316

Author: connor4312

src/vs/workbench/contrib/mcp/common/mcpSamplingService.ts

@@ -68,6 +68,10 @@ export class McpSamplingService extends Disposable implements IMcpSamplingServic
 			};
 		}).filter(isDefined);
 
+		if (opts.params.systemPrompt) {
+			messages.unshift({ role: ChatMessageRole.System, content: [{ type: 'text', value: opts.params.systemPrompt }] });
+		}
+
 		const model = await this._getMatchingModel(opts);
 		// todo@connor4312: nullExtensionDescription.identifier -> undefined with API update
 		const response = await this._languageModelsService.sendChatRequest(model, new ExtensionIdentifier('Github.copilot-chat'), messages, {}, token);