feat: define Agent and McpAgent in the same worker

[threepointone]

We want to be able to make a single worker that defines an Agent and an McpAgent, and the agent connects to the server (with this.mcp.connect())

Problems:

1. We actually want it to use a service binding to connect, but the mcp.connect api expects as URL (and there might some mcp transport constraints where it assumes fetch?)
2. Internal "fetch" won't work without the global_fetch_strictly_public flag (also it's annoying to setup the domains/host to connect to when the binding is right there)

we should solve for 1.