Revert "Remove preview from mcp setting" (microsoft#251656)

Revert "Remove preview from mcp setting (microsoft#251618)"

This reverts commit b2e66f6.

Author: joshspicer

src/vs/workbench/contrib/chat/browser/chat.contribution.ts

@@ -249,9 +249,12 @@ configurationRegistry.registerConfiguration({
 			type: 'boolean',
 			description: nls.localize('chat.mcp.enabled', "Enables integration with Model Context Protocol servers to provide additional tools and functionality."),
 			default: true,
+			tags: ['preview'],
 			policy: {
 				name: 'ChatMCP',
 				minimumVersion: '1.99',
+				previewFeature: true,
+				defaultValue: false
 			}
 		},
 		[mcpServerSamplingSection]: {