fix(translators): correct key path for system_instruction.parts in Claude request logic

internal/translator/gemini/claude/gemini_claude_request.go

@@ -56,7 +56,7 @@ func ConvertClaudeRequestToGemini(modelName string, inputRawJSON []byte, _ bool)
 			out, _ = sjson.SetRaw(out, "system_instruction", systemInstruction)
 		}
 	} else if systemResult.Type == gjson.String {
-		out, _ = sjson.Set(out, "request.system_instruction.parts.-1.text", systemResult.String())
+		out, _ = sjson.Set(out, "system_instruction.parts.-1.text", systemResult.String())
 	}
 
 	// contents