feat: serialize tool results as JSON when possible

[clareliguori]

Description

Currently tool results get serialized with str(result), which serializes objects using their Python string repr. This is difficult to parse in steering handlers and evaluators to check for result values.

This change serializes results as JSON where possible, to enable parsing message content back into dict/list or into a Pydantic model. This is backwards-compatible because it falls back to string repr.

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!