MCPRemoteProxy can't be added to an MCPGroup

## Bug description

The MCPRemoteProxy resource doesn't have a `groupRef` field like MCPServer, thus it can't be added to an MCPGroup, meaning it also can't be used with vMCP.

## Expected behavior

MCPRemoteProxy should be able to reference an MCPGroup so it can be consumed by a VirtualMCPServer resource.

## Actual behavior

MCPRemoteProxy has no `groupRef`, making it incompatible with vMCP.