[JS] need to standardize void tool input handlng across model plugins

[pavelgj]

Context: https://discord.com/channels/1255578482214305893/1309139828310540369

Here's a simple repro for OpenAI:

import { genkit, z } from 'genkit';
import { gpt4o, openAI } from 'genkitx-openai';

const ai = genkit({
  plugins: [openAI()],
  model: gpt4o,
});

const gablorken = ai.defineTool(
  {
    name: 'gablorken',
    description: 'use to calculate a gablorken',
  },
  async () => {
    return 11;
  }
);

const helloFlow = ai.defineFlow('hello', async () => {
  const { text } = await ai.generate({
    prompt: 'what is a gablorken',
    tools: [gablorken],
  });
  return text;
});

(async () => {
  console.log(await helloFlow());
})();

Throws:

  error: {
    message: `Invalid schema for function 'gablorken': schema must be a JSON Schema of 'type: "object"', got 'type: "None"'.`,
    type: 'invalid_request_error',
    param: 'tools[0].function.parameters',
    code: 'invalid_function_parameters'
  },

This works fine for GoogleAI/VertexAI/

a workaround:

const gablorken = ai.defineTool(
  {
    name: 'gablorken',
    description: 'use to calculate a gablorken',
    inputSchema: z.object({}), // <------------
  },
  async () => {
    return 11;
  }
);

[pavelgj]

/cc @mbleigh

[mbleigh]

I think we have two options here:

1. Translate "undefined input schema" to {type: "object", additionalProperties: false} when converting to a model request
2. Leave it up to the plugins to do same

I'm on the fence - it seems totally reasonable to me to have a tool with no arguments, but an empty object is probably effectively the same and will fix this problem for everyone...

Probably 2?