refactor: unify agent communication with Transport interface

- Introduce `Transport` interface to abstract WebSocket and SSH
communication
- Add generic `Data` field to `AgentResponse` for streamlined future
endpoints
- Maintain backward compatibility with legacy hubs and agents using
typed fields
- Unify fetch operations (SMART, systemd, containers) under a single
`request` method
- Improve `RequestManager` with deadline awareness and legacy response
support
- Refactor agent response routing into dedicated `agent/response.go`
- Update version to 0.18.0-beta.2

internal/hub/transport/transport.go

@@ -0,0 +1,112 @@
+// Package transport provides a unified abstraction for hub-agent communication
+// over different transports (WebSocket, SSH).
+package transport
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/fxamacker/cbor/v2"
+	"github.com/henrygd/beszel/internal/common"
+	"github.com/henrygd/beszel/internal/entities/smart"
+	"github.com/henrygd/beszel/internal/entities/system"
+	"github.com/henrygd/beszel/internal/entities/systemd"
+)
+
+// Transport defines the interface for hub-agent communication.
+// Both WebSocket and SSH transports implement this interface.
+type Transport interface {
+	// Request sends a request to the agent and unmarshals the response into dest.
+	// The dest parameter should be a pointer to the expected response type.
+	Request(ctx context.Context, action common.WebSocketAction, req any, dest any) error
+	// IsConnected returns true if the transport connection is active.
+	IsConnected() bool
+	// Close terminates the transport connection.
+	Close()
+}
+
+// UnmarshalResponse unmarshals an AgentResponse into the destination type.
+// It first checks the generic Data field (0.19+ agents), then falls back
+// to legacy typed fields for backward compatibility with 0.18.0 agents.
+func UnmarshalResponse(resp common.AgentResponse, action common.WebSocketAction, dest any) error {
+	if dest == nil {
+		return errors.New("nil destination")
+	}
+	// Try generic Data field first (0.19+)
+	if len(resp.Data) > 0 {
+		if err := cbor.Unmarshal(resp.Data, dest); err != nil {
+			return fmt.Errorf("failed to unmarshal generic response data: %w", err)
+		}
+		return nil
+	}
+	// Fall back to legacy typed fields for older agents/hubs.
+	return unmarshalLegacyResponse(resp, action, dest)
+}
+
+// unmarshalLegacyResponse handles legacy responses that use typed fields.
+func unmarshalLegacyResponse(resp common.AgentResponse, action common.WebSocketAction, dest any) error {
+	switch action {
+	case common.GetData:
+		d, ok := dest.(*system.CombinedData)
+		if !ok {
+			return fmt.Errorf("unexpected dest type for GetData: %T", dest)
+		}
+		if resp.SystemData == nil {
+			return errors.New("no system data in response")
+		}
+		*d = *resp.SystemData
+		return nil
+	case common.CheckFingerprint:
+		d, ok := dest.(*common.FingerprintResponse)
+		if !ok {
+			return fmt.Errorf("unexpected dest type for CheckFingerprint: %T", dest)
+		}
+		if resp.Fingerprint == nil {
+			return errors.New("no fingerprint in response")
+		}
+		*d = *resp.Fingerprint
+		return nil
+	case common.GetContainerLogs:
+		d, ok := dest.(*string)
+		if !ok {
+			return fmt.Errorf("unexpected dest type for GetContainerLogs: %T", dest)
+		}
+		if resp.String == nil {
+			return errors.New("no logs in response")
+		}
+		*d = *resp.String
+		return nil
+	case common.GetContainerInfo:
+		d, ok := dest.(*string)
+		if !ok {
+			return fmt.Errorf("unexpected dest type for GetContainerInfo: %T", dest)
+		}
+		if resp.String == nil {
+			return errors.New("no info in response")
+		}
+		*d = *resp.String
+		return nil
+	case common.GetSmartData:
+		d, ok := dest.(*map[string]smart.SmartData)
+		if !ok {
+			return fmt.Errorf("unexpected dest type for GetSmartData: %T", dest)
+		}
+		if resp.SmartData == nil {
+			return errors.New("no SMART data in response")
+		}
+		*d = resp.SmartData
+		return nil
+	case common.GetSystemdInfo:
+		d, ok := dest.(*systemd.ServiceDetails)
+		if !ok {
+			return fmt.Errorf("unexpected dest type for GetSystemdInfo: %T", dest)
+		}
+		if resp.ServiceInfo == nil {
+			return errors.New("no systemd info in response")
+		}
+		*d = resp.ServiceInfo
+		return nil
+	}
+	return fmt.Errorf("unsupported action: %d", action)
+}