fix: do not swap the system prompt all the time

src/BackusNaurFormGrammar.php

src/LlamaCppCompletionRequest.php

src/LlmPrompt.php

src/LlmPrompt/SubjectActionPrompt.php

src/LlmPromptTemplate.php

src/LlmPromptTemplate/ChainPrompt.php

src/LlmPromptTemplate/MistralInstructChat.php

src/LlmPromptTemplate/Phi2Question.php

src/LlmPromptTemplate/Plain.php

src/WebSocketRPCResponder/LlamaCppPromptResponder.php