refactor: Introduce standardized API error

This commit refactors the application's error handling mechanism by introducing a new standardized error type, `types.NewAPIError`. It also renames common JSON utility functions for better clarity.

Previously, internal error handling was tightly coupled to the `dto.OpenAIError` format. This change decouples the internal logic from the external API representation.

Key changes:
- A new `types.NewAPIError` struct is introduced to serve as a canonical internal representation for all API errors.
- All relay adapters (OpenAI, Claude, Gemini, etc.) are updated to return `*types.NewAPIError`.
- Controllers now convert the internal `NewAPIError` to the client-facing `OpenAIError` format at the API boundary, ensuring backward compatibility.
- Channel auto-disable/enable logic is updated to use the new standardized error type.
- JSON utility functions are renamed to align with Go's standard library conventions (e.g., `UnmarshalJson` -> `Unmarshal`, `EncodeJson` -> `Marshal`).

dto/openai_request.go

@@ -65,8 +65,8 @@ type GeneralOpenAIRequest struct {
 
 func (r *GeneralOpenAIRequest) ToMap() map[string]any {
 	result := make(map[string]any)
-	data, _ := common.EncodeJson(r)
-	_ = common.UnmarshalJson(data, &result)
+	data, _ := common.Marshal(r)
+	_ = common.Unmarshal(data, &result)
 	return result
 }