feat(tools): add toModelOutput for custom tool result formatting

[mattapperson]

Summary

• Adds optional toModelOutput function to tool definitions that converts execution output to model-facing content blocks (Responses API format)
• This decouples what calling code receives (structured data like { imageUrl }) from what the model sees (text + image content blocks)
• If toModelOutput throws, the error is surfaced (no fallback to JSON.stringify)

Example Usage

const imageGenTool = tool({
  name: 'image_gen',
  inputSchema: z.object({ prompt: z.string() }),
  execute: async ({ prompt }) => {
    const imageUrl = await generateImage(prompt);
    return { status: 'ok', imageUrl };
  },
  toModelOutput: ({ output }) => ({
    type: 'content',
    value: [
      { type: 'input_text', text: 'Image generated successfully.' },
      { type: 'input_image', detail: 'auto', imageUrl: output.imageUrl },
    ],
  }),
});

Test plan

• All existing tests pass (223 tests)
• Added 5 new unit tests for toModelOutput:
  • Tool creation with toModelOutput
  • Both output and input params received correctly
  • Async toModelOutput support
  • Tools without outputSchema
  • Generator tools with toModelOutput
• TypeScript type checking passes