Support adding MCP servers that are exposed as service bindings

[kamal]

Expose the ability to add a service binding as an MCP server via this.addMcpServer. Since the MCP server options need to be serializable, one option is to expose it as a serviceBinding option in transport.

e.g.

await this.addMcpServer("Service Binding MCP", "dummy_url", env.HOST, "agent", {
  transport: {
    type: "streamable-http",
    serviceBinding: "SERVICE_BINDING_MCP"
  }
})

The benefits would be:

1. faster
2. not consume the simultaneous open connections which might be the issue i'm seeing in streamText hanging in a multi-turn MCP session #538

Both SSEEdgeClientTransport and StreamableHTTPEdgeClientTransport support the ability to provide a custom fetch function so conceptually we would be able to pass in env[serviceBinding].fetch

[threepointone]

related: #548

[mattzcarey]

Was there a reason you were leaning towards env[serviceBinding].fetch over a service binding specific transport? I really like the transport idea since we can bypass any auth on the server. Would that work in your case?

[kamal]

No particularly reason except to make it transparent to the MCP sdk.

I am not so sure about being able to bypass auth. My MCP server is very dependent on Props being hydrated. I remember it was important that we presented oauthProvider and/or McpAgent the internal auth token for it to fetch the props from the store to rehydrate.