Add json_response_mode instead of structured_output

[dirkbrnd]

Description

This addresses a confusing DX point where users often have to set response_model and structured_output.

This changes the behaviour to default to structured output if supported.

json_response_mode can be activated if needed.

---

Type of change

Please check the options that are relevant:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Model update (Addition or modification of models)
• Other (please describe):

---

Checklist

• Adherence to standards: Code complies with Agno’s style guidelines and best practices.
• Formatting and validation: You have run ./scripts/format.sh and ./scripts/validate.sh to ensure code is formatted and linted.
• Self-review completed: A thorough review has been performed by the contributor(s).
• Documentation: Docstrings and comments have been added or updated for any complex logic.
• Examples and guides: Relevant cookbook examples have been included or updated (if applicable).
• Tested in a clean environment: Changes have been tested in a clean environment to confirm expected behavior.
• Tests (optional): Tests have been added or updated to cover any new or changed functionality.

---

Additional Notes

Include any deployment notes, performance implications, security considerations, or other relevant information (e.g., screenshots or logs if applicable).