fix: do not request the LLM to provide a $defs field

[nlenepveu]

Issue

When using output_model with nested objects, an extra $defs field is listed in the json fields requested in the output. This field contains the definitions of nested objects, but it should not be included as a top-level field in the output JSON structure.

Provide your output as a JSON containing the following fields:
<json_fields>
["entities", "$defs"] // HERE
</json_fields>
Here are the properties for each field:
<json_field_properties>
{
  "entities": {
    "description": "The list of entities.",
    "items": {
      "$ref": "#/$defs/EntityOutput"
    },
    "type": "array"
  },
  "$defs": {
    "EntityOutput": {
      "name": {
        "description": "The name of the category as output by the category lookup
tool.",
        "type": "string"
      }
    }
  }
}
</json_field_properties>

Expected behavior

Even if the $defs field must be present in the json field properties description, it should not be listed in the json fields requested in the output.

Provide your output as a JSON containing the following fields:
<json_fields>
["entities"]
</json_fields>
Here are the properties for each field:
<json_field_properties>
{
  "entities": {
    "description": "The list of entities.",
    "items": {
      "$ref": "#/$defs/EntityOutput"
    },
    "type": "array"
  },
  "$defs": {
    "EntityOutput": {
      "name": {
        "description": "The name of the category as output by the category lookup tool.",
        "type": "string"
      }
    }
  }
}
</json_field_properties>

[ashpreetbedi]

@nlenepveu quick check if this breaks nested output models?

[ashpreetbedi]

will approve and merge so can test this locally