Add length check logic for lists of fixed length

[jopemachine]

What Operating System(s) are you seeing this problem on?

Linux (x86-64)

Backend.AI version

main

Describe the bug

For lists that must have a length of 1 (or other fixed value), it would be better to include separate length-checking logic.
See #3137 (comment) for reference.
IMHO it would be better to fix issues as they come up, rather than trying to fix everything in the code at once.