[PYTHON] Fix for failing to lookup discriminator value using AllOf and discriminator

[zishunwei]

Fix issue https://github.com/OpenAPITools/openapi-generator/issues/18495

For discriminator, it should be modelName instead of 'mappingName' in from_dict method in the template since object_type has been the mapped name (modelName) by get_discriminator_value()

    @classmethod
    def from_dict(cls, obj: dict) -> {{^hasChildren}}{{{classname}}}{{/hasChildren}}{{#hasChildren}}{{#discriminator}}Union({{#children}}{{{classname}}}{{^-last}}, {{/-last}}{{/children}}){{/discriminator}}{{^discriminator}}{{{classname}}}{{/discriminator}}{{/hasChildren}}:
        """Create an instance of {{{classname}}} from a dict"""
        {{#hasChildren}}
        {{#discriminator}}
        # look up the object type based on discriminator mapping
        object_type = cls.get_discriminator_value(obj)
        {{#mappedModels}}
        if object_type ==  '{{{modelName}}}':
            return import_module("{{packageName}}.models.{{model.classVarName}}").{{modelName}}.from_dict(obj)
        {{/mappedModels}}
        raise ValueError("{{{classname}}} failed to lookup discriminator value from " +
                            json.dumps(obj) + ". Discriminator property name: " + cls.__discriminator_property_name +
                            ", mapping: " + json.dumps(cls.__discriminator_value_class_map))

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[zishunwei]

Hi @wing328 @fa0311 , could you please have a review if you've time? Thanks