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/proxmox/config.go

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