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`).

service/convert.go

@@ -163,7 +163,7 @@ func ClaudeToOpenAIRequest(claudeRequest dto.ClaudeRequest, info *relaycommon.Re
 						oaiToolMessage.SetStringContent(mediaMsg.GetStringContent())
 					} else {
 						mediaContents := mediaMsg.ParseMediaContent()
-						encodeJson, _ := common.EncodeJson(mediaContents)
+						encodeJson, _ := common.Marshal(mediaContents)
 						oaiToolMessage.SetStringContent(string(encodeJson))
 					}
 					openAIMessages = append(openAIMessages, oaiToolMessage)