Remove additionalProperties from the JSON response schema

[dkotter]

What?

Closes #17

Ensure we remove additionalProperties from JSON response schemas

Why?

If using the AI Client to make requests and you set a JSON response schema, OpenAI requires the additionalProperties key but Google will throw an error if this exists. We already remove that key from function declarations but we also need to remove it from JSON schemas

How?

• Ensure the response schema is passed through the existing removeAdditionalPropertiesKey method
• Update that method to process the items key, both top-level and nested

Use of AI Tools

Used Cursor running GPT-5.3 Codex to initially diagnose the problem (was reported on the WordPress AI plugin). Took the suggested fix, tested that and cleaned it up

Testing Instructions

Hard to test with this package alone, recommend testing with the AI plugin:

1. Install the AI plugin
2. Set the Google Provider up
3. Turn on the Review Notes Experiment
4. Trigger that on a post
5. Ensure it works correctly and no errors are shown in the console

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[felixarntz]

@dkotter Great catch!