No one promised that NATO will not expand, and it is not documented. It was discussed, but that’s it. It is Russia-propagated myth.
Gorbachev, whose ambiguous statement about a feeling of betrayal when NATO did expand in the east beyond the reunited Germany is the source of the myth of the promise, later explicitly stated that no promise was made and the issue was never even brought up.
(And it's not as if the USSR and the USA didn’t understand the use of memorializing important agreements in writing to prevent later disputes about their scope and content.)
[0] https://www.latimes.com/opinion/op-ed/la-oe-shifrinson-russi...
That just does not prove NATO agreed not to expand.
on the other hand, russia was attacked multiple times by western european countries and lost millions of its citizens. i think their unease at an alliamce which you yourself say is hostile, is justified
But whether or not it was open for discussion, no commitment was, in fact, made and Gorbachev has himself—after inspiring the myth by a vague statement about a feeling of betrayal at NATO expansion—acknowledged that and gone further and saying that a commitment on the issue was never even discussed as part of the negotiations.
Saying something could be part of a deal as an inducement to get someone to the negotiating table doesn't automatically insert it into the deal, or even guarantee it gets talked about in the negotiation.