feat: enhance API and session management with Nacos and Redis integration

- Add Nacos registry for service registration and deregistration.
- Implement Redis registry for session management with heartbeat and session claiming.
- Improve completion service with session handling and request validation.
- Enhance WebSocket handling for completion requests with JSON-RPC support.
- Add tests for new registry implementations and completion manager functionalities.
- Refactor existing code for better readability and maintainability.

backend/internal/api/handler_test.go

@@ -28,6 +28,7 @@ func (f *fakeCompletionService) Complete(_ context.Context, _ completion.Request
 	return f.resp, nil
 }
 
+// 验证 HTTP 补全接口的成功路径。
 func TestRegisterRoutesCompletionSuccess(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	r := gin.New()
@@ -63,6 +64,7 @@ func TestRegisterRoutesCompletionSuccess(t *testing.T) {
 	}
 }
 
+// 验证非法 JSON 会返回 400。
 func TestRegisterRoutesCompletionBadJSON(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	r := gin.New()
@@ -78,6 +80,7 @@ func TestRegisterRoutesCompletionBadJSON(t *testing.T) {
 	}
 }
 
+// 验证业务校验错误会映射为 400。
 func TestRegisterRoutesCompletionValidationError(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	r := gin.New()
@@ -100,6 +103,7 @@ func TestRegisterRoutesCompletionValidationError(t *testing.T) {
 	}
 }
 
+// 验证未知内部错误会映射为 500。
 func TestRegisterRoutesCompletionServerError(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	r := gin.New()
@@ -121,6 +125,7 @@ func TestRegisterRoutesCompletionServerError(t *testing.T) {
 	}
 }
 
+// 验证 WebSocket 补全协议的基础成功流程。
 func TestRegisterRoutesCompletionWebSocketSuccess(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	r := gin.New()