Refactor `flobject.get_cls`

[seanpearsonuk]

@prmukherj @mkundu1
Overall, get_cls mixes multiple separable concerns and has a cyclomatic complexity above the range usually considered "high". We should refactor the code and add all the necessary tests. Copilot can assist. This is separate work, and I am creating an issue from this comment.

Originally posted by @seanpearsonuk in #5126