refactor: replace skill code with skill ID across the application

- Updated skill handling to use skill IDs instead of skill codes in various components, services, and models.
- Modified tests to reflect changes in skill identification.
- Removed references to skill codes in favor of skill IDs for consistency and clarity.
- Updated localization files to remove skill code references and adjust error messages accordingly.

internal/handlers/dashboard/ai_agent_handler.go

@@ -215,7 +215,6 @@ func buildAIAgentResponseWithLocale(item *models.AIAgent, locale string) respons
 		if skill := services.SkillDefinitionService.Get(id); skill != nil {
 			ret.Skills = append(ret.Skills, response.AIAgentSkillResponse{
 				ID:   skill.ID,
-				Code: skill.Code,
 				Name: skill.Name,
 			})
 		}