feat(js): use predefined agent's models on multi-agent systems

[marioToribi0]

Description here... Help the reviewer by:

🐞 Observed Behavior (Current Limitation)
In the current beta API for multi-agent systems, when an orchestrator agent invokes a sub-agent (or a nested agent), the model specified in the sub-agent's configuration is ignored. The model used for the sub-agent's execution is always inherited from the parent/orchestrator agent's execution context.

• The model is explicitly defined on the sub-agent using the model property within ai.definePrompt, as shown below:

export const agent = ai.definePrompt({
  name: 'triageAgent',
  description: 'Triage Agent',
  model: "googleai/gemini-2.5-flash", // <- This model keeps for all the subagents
  tools: [catalogAgent, paymentAgent, representativeAgent],
  system: `You are an AI customer service agent for TechStore Computer Shop.
    Greet the user and ask them how you can help. Route them to the appropriate specialist agent:
    - Use catalogAgent for browsing products, searching items, or getting product information
    - Use paymentAgent for payment processing
    - Use representativeAgent for providing store information
    If you cannot help the customer with the available tools, politely explain so.`,
});

export const paymentAgent = ai.definePrompt({
    name: 'paymentAgent',
    model: "googleai/gemini-2.5-flash-lite", // <-- This model is currently being overwritten
    description:
      'Payment Agent can help process payments',
    tools: [processPaymentTool],
    system: `You are a payment agent for TechStore Computer Shop.
      Help customers process payments.
      Always confirm payment details before processing and provide clear transaction information.`,
  });

• With this improvement, each agent (orchestrator and sub-agents) should respect its own configured model property. If the model is not defined on the sub-agent, it should inherit from the parent agent, or fall back to the project default.
• Also, I add it a test app to test this project and futures features for multi-agent systems.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

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[pavelgj]

thanks for the PR. It looks good. Could you please run pnpm format to fix the formatter check.