fix(translator): add fallback for system_instruction key in Gemini request parsing

- Added fallback to handle cases where `system_instruction.parts` is absent by checking for `systemInstruction.parts`.

internal/translator/codex/gemini/codex_gemini_request.go

@@ -86,6 +86,9 @@ func ConvertGeminiRequestToCodex(modelName string, inputRawJSON []byte, _ bool)
 
 	// System instruction -> as a user message with input_text parts
 	sysParts := root.Get("system_instruction.parts")
+	if !sysParts.Exists() {
+		sysParts = root.Get("systemInstruction.parts")
+	}
 	if sysParts.IsArray() {
 		msg := []byte(`{"type":"message","role":"developer","content":[]}`)
 		arr := sysParts.Array()