[JS] model specified in .prompt is ignored

[kevinthecheung]

Describe the bug
the model specified in .prompt is ignored

To Reproduce

This .prompt file:

---
model: googleai/gemini-1.5-pro
---
You are the world's most welcoming AI assistant. Greet the user and offer your assistance.

Fails when run with:

const ai = genkit({
  plugins: [
    googleAI(),
  ],
});

const helloDotPrompt = ai.defineFlow(
    { name: "helloP" },
    async () => {
        const helloPrompt = await ai.prompt('hello');
        const { text } = await helloPrompt();
        return text;
    }
)

console.log(await helloDotPrompt());

Error:

node:internal/modules/run_main:129
    triggerUncaughtException(
    ^

Error: Model is required.
    at /Users/kevincheung/tmp/0.9-dev.1-test/node_modules/.pnpm/@genkit-ai+ai@0.9.0-dev.2/node_modules/@genkit-ai/ai/lib/generate.js:396:13
    at Generator.next (<anonymous>)
    at /Users/kevincheung/tmp/0.9-dev.1-test/node_modules/.pnpm/@genkit-ai+ai@0.9.0-dev.2/node_modules/@genkit-ai/ai/lib/generate.js:54:61
    at new Promise (<anonymous>)
    at __async (/Users/kevincheung/tmp/0.9-dev.1-test/node_modules/.pnpm/@genkit-ai+ai@0.9.0-dev.2/node_modules/@genkit-ai/ai/lib/generate.js:38:10)
    at resolveModel (/Users/kevincheung/tmp/0.9-dev.1-test/node_modules/.pnpm/@genkit-ai+ai@0.9.0-dev.2/node_modules/@genkit-ai/ai/lib/generate.js:393:10)
    at /Users/kevincheung/tmp/0.9-dev.1-test/node_modules/.pnpm/@genkit-ai+ai@0.9.0-dev.2/node_modules/@genkit-ai/ai/lib/generate.js:433:33
    at Generator.next (<anonymous>)
    at fulfilled (/Users/kevincheung/tmp/0.9-dev.1-test/node_modules/.pnpm/@genkit-ai+ai@0.9.0-dev.2/node_modules/@genkit-ai/ai/lib/generate.js:41:24)

Adding a default model to genkit() makes it work.

Expected behavior
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Screenshots
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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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