Preserve empty JSON schema metadata

[biefan]

Summary

• preserve explicitly provided empty JSON schema objects in _JsonResponseConfig.from_metadata()
• add a regression test covering json_schema={} metadata

Problem

_JsonResponseConfig.from_metadata() currently treats json_schema={} as falsy and falls through to the same branch as “no schema provided”. That means these two inputs behave differently even though they both represent an explicit empty schema:

• json_schema="{}" -> preserved as {}
• json_schema={} -> silently converted to None

This is an inconsistent metadata parsing bug rather than a validation decision, because the string and dict forms should not diverge like this.

Testing

• .venv/bin/pytest tests/unit/models/test_json_response_config.py -q