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`).

common/json.go

@@ -5,7 +5,7 @@ import (
 	"encoding/json"
 )
 
-func UnmarshalJson(data []byte, v any) error {
+func Unmarshal(data []byte, v any) error {
 	return json.Unmarshal(data, v)
 }
 
@@ -17,6 +17,6 @@ func DecodeJson(reader *bytes.Reader, v any) error {
 	return json.NewDecoder(reader).Decode(v)
 }
 
-func EncodeJson(v any) ([]byte, error) {
+func Marshal(v any) ([]byte, error) {
 	return json.Marshal(v)
 }