In 'The window is broken', 'broken' MUST be an adjective. It is an adjective describing the state of the window and is the same structure as 'the window is dirty'. If it were passive voice, it would be possible to make an active sentence. It is not. '??? breaks the window' - this has vital information missing (who, when) and does not carry the same meaning as the original. It's the same with your additional sentence. BTW - In 'The window is broken', 'is' (be) is not an auxiliary verb. It is the main and only verb in the sentence. 'Be' is only an auxiliary when used with another verb to form other tenses or moods, e.g. Present Continuous, Passive Voice. If the sentence were 'the window has been broken' or 'the window is being broken', these would be Passive Voice' because we could also say 'somebody has broken the window' or 'somebody is breaking the window' and the meaning conveyed would be the same.

I would say it would be a linking verb "are" and an adjective "broken". If it were passive it would be more "The windows are being broken". I hope that answers your question.

Another example: He is trapped. Present simple passive voice or auxiliary verb(be-->is)+adjective(trapped)?