Ensure a common healthcheck for the proxy

[JAORMX]

Currently, ToolHive supports two types of MCP servers:

• stdio
• sse (HTTP SSE)

For each type of server, there is a different type of proxy to front it:

• HTTP SSE: Takes HTTP SSE requests and forwards them to an stdio server
• transparent: Takes a request as-is and forwards it to an HTTP SSE server.

The HTTP SSE proxy has a healthcheck, while the transparent proxy does not.

We should add a healthcheck to the transparent proxy, and ensure it returns the same format as the HTTP SSE proxy.