fix: better cli error when missing mcp dep (#6859)

<img width="898" height="173" alt="Screenshot 2025-10-21 at 10 08 47 AM"
src="https://github.com/user-attachments/assets/805bdb35-6b75-4f99-8d86-3ac1ada7c4d3"
/>

Author: mscolnick

marimo/_mcp/server/main.py

@@ -12,6 +12,7 @@
 
 from marimo._ai._tools.base import ToolContext
 from marimo._ai._tools.tools_registry import SUPPORTED_BACKEND_AND_MCP_TOOLS
+from marimo._dependencies.dependencies import DependencyManager
 from marimo._loggers import marimo_logger
 
 LOGGER = marimo_logger()
@@ -33,6 +34,12 @@ def setup_mcp_server(app: Starlette) -> None:
     Returns:
         StreamableHTTPSessionManager: MCP session manager
     """
+    if not DependencyManager.mcp.has():
+        from click import ClickException
+
+        msg = "MCP dependencies not available. Install with `pip install marimo[mcp]` or `uv add marimo[mcp]`"
+        raise ClickException(msg)
+
     from mcp.server.fastmcp import FastMCP
     from starlette.middleware.base import BaseHTTPMiddleware
     from starlette.responses import JSONResponse