refactor: propagate context and standardize HTTP client timeouts

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.

internal/agentpool/agent.go

@@ -27,6 +27,7 @@ func newAgent(cfg *agent.AgentConfig) *Agent {
 		AgentConfig: cfg,
 		httpClient: &http.Client{
 			Transport: transport,
+			Timeout:   5 * time.Second,
 		},
 		fasthttpHcClient: &fasthttp.Client{
 			DialTimeout: func(addr string, timeout time.Duration) (net.Conn, error) {