JSON output support from tool calls

The current tool response is in Markdown format, which is a really nice improvement you guys have made, and I fully understand the rationale behind the move.
However, with Markdown output, it's quite difficult to render frontend components from the tool response (e.g., in Gen UI).

If each tool could accept an outputFormat parameter (with literal values like "md" or "json"), it would solve this issue. This parameter should be optional and default to "md", but if set to "json", the tool must return the raw JSON API response instead of the parsed Markdown output.

I'm open to creating a PR for this as well.