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`).

model/channel.go

@@ -54,13 +54,13 @@ type ChannelInfo struct {
 
 // Value implements driver.Valuer interface
 func (c ChannelInfo) Value() (driver.Value, error) {
-	return common.EncodeJson(&c)
+	return common.Marshal(&c)
 }
 
 // Scan implements sql.Scanner interface
 func (c *ChannelInfo) Scan(value interface{}) error {
 	bytesValue, _ := value.([]byte)
-	return common.UnmarshalJson(bytesValue, c)
+	return common.Unmarshal(bytesValue, c)
 }
 
 func (channel *Channel) getKeys() []string {