fix: correct get_user_cache fuction and add tests

[laraujo7]

What kind of change does this PR introduce?

• Code imrpovements
• Test improvements

What is the new behavior?

• Renamed get_user_cache/4 to fetch_user_cache/4 to better reflect its return value ({:ok, ...} | {:error, ...}).
• Fixed behavior where errors were being cached — these are now ignored instead of stored.
• Added missing test coverage for both :single and :cluster modes of fetch_user/4.