oneOf with boolean offers implicit bool conversion

[idg10]

We have this type:

    "stringOrIsAvailable": {
      "oneOf": [
        { "type": "boolean" },
        { "type": "string" }
      ]
    },

(In the original context, the first type was actually { "$ref": "#/$defs/isAvailable" }, but isAvailable is just {"type":"boolean"}

The generated code defines an implicit conversion to bool, which fails with an InvalidOperationException if it's not a boolean. We were expecting this to be an explicit conversion.