Fix pydantic-ai deprecation warning: mcp_servers -> toolsets (#1206)

Updates the pydantic-ai Agent instantiation to use the new 'toolsets'
parameter instead of the deprecated 'mcp_servers' parameter. This change
is internal only - Marvin's public API remains unchanged.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: zzstoatzz

src/marvin/agents/agent.py

@@ -206,7 +206,7 @@ async def get_agentlet(
             agent_kwargs["tools"] = combined_tools
 
         if active_mcp_servers:
-            agent_kwargs["mcp_servers"] = active_mcp_servers
+            agent_kwargs["toolsets"] = active_mcp_servers
 
         agentlet = pydantic_ai.Agent[Any, Any](**agent_kwargs)