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) {

internal/health/check/http.go

@@ -76,8 +76,11 @@ func H2C(ctx context.Context, url *url.URL, method, path string, timeout time.Du
 
 	setCommonHeaders(req.Header.Set)
 
+	client := *h2cClient
+	client.Timeout = timeout
+
 	start := time.Now()
-	resp, err := h2cClient.Do(req)
+	resp, err := client.Do(req)
 	lat := time.Since(start)
 
 	if resp != nil {

internal/proxmox/config.go

@@ -162,4 +162,4 @@ func (c *Config) refreshSessionLoop(ctx context.Context) {
 			}
 		}
 	}
-}
+}