[JS] System prompt error after restoring chat session

[kevinthecheung]

Describe the bug

After restoring a chat session the system prompt causes the following error

Error: system role is not supported
    at toGeminiRole (/Users/kevincheung/gitrepos/genkit/js/plugins/googleai/src/gemini.ts:184:15)
    at toGeminiMessage (/Users/kevincheung/gitrepos/genkit/js/plugins/googleai/src/gemini.ts:391:11)
    at <anonymous> (/Users/kevincheung/gitrepos/genkit/js/plugins/googleai/src/gemini.ts:588:29)
    at Array.map (<anonymous>)
    at <anonymous> (/Users/kevincheung/gitrepos/genkit/js/plugins/googleai/src/gemini.ts:588:12)
    at Generator.next (<anonymous>)
    at /Users/kevincheung/gitrepos/genkit/js/plugins/googleai/lib/gemini.js:66:61
    at new Promise (<anonymous>)
    at __async (/Users/kevincheung/gitrepos/genkit/js/plugins/googleai/lib/gemini.js:50:10)
    at <anonymous> (/Users/kevincheung/gitrepos/genkit/js/plugins/googleai/src/gemini.ts:509:43)

To Reproduce

Test app:

class JsonSessionStore<S = any> implements SessionStore<S> {
  async get(sessionId: string): Promise<SessionData<S> | undefined> {
    try {
      const s = await readFile(`${sessionId}.json`, { encoding: 'utf8' });
      const data = JSON.parse(s);
      return data;
    } catch {
      return undefined;
    }
  }

  async save(sessionId: string, sessionData: SessionData<S>): Promise<void> {
    const s = JSON.stringify(sessionData);
    await writeFile(`${sessionId}.json`, s, { encoding: 'utf8' });
  }
}

(async () => {
  let session;
  try {
    const s = await readFile('sessionId.txt', { encoding: 'utf8' });
    const sessionId = s.trim();
    session = await ai.loadSession(sessionId, {
      store: new JsonSessionStore(),
    });
  } catch {
    session = ai.createSession({
      store: new JsonSessionStore(),
    });
    await writeFile('sessionId.txt', session.id, { encoding: 'utf8' });
  }
  const chat = session.chat({
    model: gemini15Pro,
    system:
      "You're a pirate first mate. Address the user as Captain and assist " +
      'them however you can.',
    config: {
      temperature: 1.3,
    },
  });

  console.log("You're chatting with Gemini. Ctrl-C to quit.\n");
  const readline = createInterface(process.stdin, process.stdout);
  while (true) {
    const userInput = await readline.question('> ');
    const { text } = await chat.send(userInput);
    console.log(text);
  }
})();

Run the program, create some chat history, control C to quit, run the program again and try to send a message

Expected behavior
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Runtime (please complete the following information):

• OS: [e.g. Linux, MacOS]
• Version [e.g. 22]

** Node version

• run node --version at paste here

Additional context
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