fix: use json.RawMessage for Instructions field in OpenAIResponsesResponse (#4260)

The Instructions field in OpenAIResponsesResponse was defined as string,
but upstream providers may return null or non-string JSON values for this
field. This causes json.Unmarshal to fail, resulting in HTTP 500 on
/v1/responses endpoint.

Other fields in the same struct (Status, ToolChoice, Truncation, etc.)
already use json.RawMessage. The request-side DTO (openai_request.go)
also defines Instructions as json.RawMessage. This fix aligns the
response-side with both patterns.

Co-authored-by: 40005415C\Administrator <linbin@envicool.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

dto/openai_response.go

@@ -272,7 +272,7 @@ type OpenAIResponsesResponse struct {
 	Status             json.RawMessage    `json:"status"`
 	Error              any                `json:"error,omitempty"`
 	IncompleteDetails  *IncompleteDetails `json:"incomplete_details,omitempty"`
-	Instructions       string             `json:"instructions"`
+	Instructions       json.RawMessage    `json:"instructions"`
 	MaxOutputTokens    int                `json:"max_output_tokens"`
 	Model              string             `json:"model"`
 	Output             []ResponsesOutput  `json:"output"`