Fix multiple issues with converting type hints to JSON Schema

[kepler]

Description

Summary of changes:

The main problem: newish syntax for unions with | supported by Python 3.10 are of type types.UnionType, and didn't match Union. There's a very recently completed issue on CPython that will unify both (apparently for 3.14). See also this MyPy warning about evaluating type hints.

Other problems:

• Only unions of two values were properly handled for optional values;
• Unions with more than two values and an optional one would only parse one of the values, dropping the others;
• Some code lines were never true (like isinstance(arg_json_schema["type"], list));

The code is now simplified while handling new type annotation syntax and null/optional values.

• Related issues: I didn't find any.
• Motivation and context: I was getting errors with a custom toolkit that uses the new type hinting syntax.
• Environment or dependencies: None.
• Impact on metrics: N/A.

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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):

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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.

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Additional Notes

Linked in the description.