Use try/hash() to detect uncacheable args in process_cached

[Chessing234]

Bug

process_cached.__call__ crashes with TypeError: unhashable type: 'list' when the decorated function is invoked with an unhashable
argument, even though the code has an explicit guard intended to fall
back to an uncached call in that exact case.

Root cause

def __call__(self, *args):
    if not isinstance(args, collections.abc.Hashable):
        # uncacheable. a list, for instance.
        # better to not cache than blow up.
        return self.func(*args)
    if args in self.cache:
        ...

args is always a tuple (Python's *args collection), and
tuple is registered as collections.abc.Hashable. isinstance(..., Hashable) therefore answers "yes, hashable" for any tuple, including
tuples whose elements are not actually hashable (([1, 2, 3],),
({"k": 1},), etc.). The guard passes, execution continues into
args in self.cache, and the dict lookup attempts
hash(args) - which recurses into the list element and raises
TypeError.

The comment ("uncacheable. a list, for instance. better to not cache
than blow up.") documents the intended behaviour, so the bug is
specifically in the check, not the fallback.

Why the fix is correct

• hash(args) is what args in self.cache will call internally, so
  attempting it in a try is the faithful probe: if the lookup would
  crash, so does this, and we fall through to the uncached
  return self.func(*args) path.
• Python's own documentation (and the Hashable ABC docs) explicitly
  call out this tuple-of-list case as the reason isinstance(..., Hashable) is insufficient.
• No behavioural change when args is truly hashable: hash(args)
  succeeds, the caching path runs as before.
• Drops the now-unused import collections.

Change

openedx/core/lib/cache_utils.py: replace the isinstance(..., collections.abc.Hashable) check in process_cached.__call__ with a
try: hash(args) / except TypeError: guard; remove the unused
collections import.

[openedx-webhooks]

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[mphilbrick211]

Hi @Chessing234! As mentioned in the other pull request you opened, please make sure to submit a CLA form so we can get this contribution review for you. Thanks!