Do the texts of Mahayana Buddhism or those of any other school posit that everything is to be regarded as ideation? (Idealism) Does Mahayana Buddhism or any other school of Buddhism posit the idea that the physical world does not exist apart from an observer's frame of reference, as in everything exists only in the mind as imagination?
the idea that the physical world does not exist apart from an observer's frame of reference

This is agreeable for expression. As i understand it, the Theravadin texts posit this iow.

as in everything exists only in the mind as imagination

This is dodgy. Which is closer to what you are asking?

That which imagines is imagined.
That which imagines isn't imagined.

There is a fundamental difference and #1 is closer to Theravada. #2 would be disagreeable.