Ukraine

The Russians don't want to get immediately destroyed, so they let it slide, that's how.

They say that about everything NATO does.

Why should repairing vehicles be any more of a casus belli, then providing it in the first place. Or things like providing intel or training soldiers? The UK for example is providing long range storm shadow cruise missiles that can be programmed to target a specific objective, presumably training how to use it, and maybe even intel that leads to target selection. At that point one could argue that the only major difference is who presses the button.

I think it boils down to the fact that there is no strict definition what counts as a cases belli, but it rather is a choice whether or not someone declares it one. If they wanted to I am sure both sides Russia and NATO could declare a number of instances as such, but they choose not to, since neither party is interested in a direct confrontation.

Because that would have serious consequences which might even come down to nuclear weapons, considering the imbalance of strength in conventional military. With NATO (especially the US) vastly outclassing Russia.

Because Russia doesn’t want to lose? They can’t even handle Ukraine with hand me downs.

If they don't have a casus belli to be in Ukraine in the first place it's hard to see how they could bitch about this

Because Russia (and indeed anybody else) doesn't want open war with anybody they don't think they can beat.

In most cases the cold war continues in proxy form.

Let them come

In theory if Russia wanted to declare a war on the United States, it could probably use "they repaired (and indeed, sent in the first place) these pieces of enemy military equipment" as a justification to their population. It might not be as effective a justification as something like a direct attack from the US on Russian troops would be, but it'd probably be more effective at gaining public support than declaring such a war for no reason and maybe anger uninvolved parties slightly less, so in that sense it could be called a casus belli. However, having a casus belli does not require one to declare a war, and if war is desired, one can probably be fabricated anyway. Russia knows that the consequences of a direct war will the US would be disastrous at best and risk the very existence of their country at worst, depending on how far the conflict escalated, so it will not actually declare such a war over something like this, even if it technically could

Russia doesn't want its front teeth kicked in?