fix: only pass enable_structured_output_with_tools

 when enabled

Maintain backward compatibility with third-party Model implementations by
only passing the enable_structured_output_with_tools parameter when it's
explicitly enabled (True). This prevents TypeErrors in custom Model classes
that don't support the new parameter yet.

- Built-in models have default False, so they work either way
- Third-party models without the parameter won't crash
- Feature still works when explicitly enabled

Fixes backward compatibility issue raised in code review.

Author: devv-shayan