Update simple mcp server to use stateless_http

[mconflitti-pbc]

https://github.com/posit-dev/connect-extensions/blob/main/extensions/simple-mcp-server/main.py#L13

For some local agents that try to connect to an MCP server on connect, it needs to be using stateless_http = true when init'd

[nealrichardson]

https://gofastmcp.com/deployment/http#enabling-stateless-mode looks like it's a simple arg to add in the constructor, or also settable by env var. (Possibly related, we should make sure the example works with the 3.0 release of FastMCP.)