feat(js/ai): added support for tools that can return multipart responses

[pavelgj]

      const t = ai.defineTool(
        { name: 'multipart-tool', description: 'description', multipart: true },
        async () => ({
          output: 'tool called',
          content: [{ text: 'part 1' }],
        })
      );

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)

[pavelgj]

PR train: #3807