refactor(agent): improve agent.go structure and documentation

- Restructure code to enhance readability and maintainability
- Add comprehensive comments to clarify interface and method purposes
- Introduce new constructor functions for Agent interface:
  - NewAgent: supports both HTTP and gRPC
  - NewAgentWithGRPC: supports only gRPC
  - NewAgentWithHTTP: supports only HTTP
- Remove unused const declarations

agent/agent.go

@@ -10,47 +10,62 @@ import (
 	"google.golang.org/grpc"
 )
 
-const (
-	ApiVersionV1   = "/api/v1"
-	DefaultPrefix  = "/api"
-	DefaultVersion = "v1"
-)
+// ApiVersionV1 defines the version number of the API
+const ApiVersionV1 = "/api/v1"
+
+// DefaultPrefix defines the default API prefix
+const DefaultPrefix = "/api"
+
+// DefaultVersion defines the default API version
+const DefaultVersion = "v1"
 
+// Agent is an interface that combines the HTTPAgent and GRPCAgent interfaces
 type Agent interface {
 	HTTPAgent
 	GRPCAgent
 }
 
+// HTTPAgent is an interface that defines the basic methods of an HTTP proxy
 type HTTPAgent interface {
+	// URI returns the URI of the HTTP service
 	URI() string
+	// HTTPServer returns an instance of the HTTP server
 	HTTPServer() *http.Server
+	// Route returns an instance of the HTTP router
 	Route() *http.Router
 }
 
+// GRPCAgent is an interface that defines the basic methods of a gRPC proxy
 type GRPCAgent interface {
+	// Server returns an instance of the gRPC server
 	Server() *grpc.Server
+	// RegisterService registers a gRPC service
 	RegisterService(desc *grpc.ServiceDesc, impl interface{})
 }
 
+// agent is an implementation of the Agent interface
 type agent struct {
 	GRPCAgent
 	HTTPAgent
 }
 
+// NewAgent creates a new Agent instance that supports both HTTP and gRPC
 func NewAgent(server *http.Server, grpcServer *grpc.Server) Agent {
 	return &agent{
 		GRPCAgent: NewGRPC(grpcServer),
 		HTTPAgent: NewHTTP(server),
 	}
 }
 
+// NewAgentWithGRPC creates a new Agent instance that only supports gRPC
 func NewAgentWithGRPC(grpcServer *grpc.Server) Agent {
 	return &agent{
 		GRPCAgent: NewGRPC(grpcServer),
 		HTTPAgent: UnimplementedAgent,
 	}
 }
 
+// NewAgentWithHTTP creates a new Agent instance that only supports HTTP
 func NewAgentWithHTTP(server *http.Server) Agent {
 	return &agent{
 		GRPCAgent: UnimplementedAgent,