[Go] fix validation of property names when a model has required fields and doesn't allow additional properties

[ctreatma]

This updates the Go client generator so that, if a model has required fields but does not allow additional properties, the custom UnmarshalJSON function will return an error when the input JSON includes additional properties.

Fixes #17156

cc/ @antihax @grokify @kemokemo @jirikuncar @ph4r5h4d @lwj5 @wing328

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