Add context parameter to TCP/UDP stream health checks and client constructors
for proper cancellation and deadline propagation. Switch from encoding/json
to sonic for faster JSON unmarshaling.
Standardize HTTP client timeouts to 5 seconds
across agent pool and health check.
* **New Features**
* Multiplexed TLS port: HTTP API and a custom stream protocol can share one port via ALPN.
* Agent-side TCP and DTLS/UDP stream tunneling with health-check support and runtime capability detection.
* Agents now advertise per-agent stream support (TCP/UDP).
* **Documentation**
* Added comprehensive stream protocol documentation.
* **Tests**
* Extended integration and concurrency tests covering multiplexing, TCP/UDP streams, and health checks.
* **Chores**
* Compose/template updated to expose both TCP and UDP ports.
