[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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[dahu33]

This PR added a critical bug described here: #21164

Also, if I may comment here, it is not because additionalProperties is set to false in the OpenAPI spec that a generated client should completely fail if the server returns extra fields. In real-world APIs, it is common for servers to evolve and add new properties over time.

The use of DisallowUnknownFields() in generated client code is extremely dangerous because:

• It prevents clients from tolerating new fields added to the server response, making clients extremely fragile.
• It breaks backward compatibility: older clients interacting with newer servers will crash when they encounter unexpected fields.
• It fundamentally contradicts robust client design principles, where clients should ignore unknown fields rather than fail.

I would propose to remove DisallowUnknownFields() from the Go client templates as soon as possible to restore proper resilience and compatibility when dealing with evolving APIs (and embedded structs).