Streamable HTTP with Health Check

Location: crates/rust-mcp-sdk/examples/streamable_http_healthcheck.rs

Demonstrates enabling the health check endpoint with a custom JSON health handler.

cargo run --example streamable_http_healthcheck -p rust-mcp-sdk

$ curl http://127.0.0.1:8080/health
{"status":"healthy","uptime_secs":42,"connections":1}