This commit refactors the logic used to search for unfinished file
uploads in the bucket, primarily focusing on the
`_find_unfinished_file_by_plan_id`, `_match_unfinished_file_if_possible`,
and `_find_matching_unfinished_file` functions.

Key changes include:

- Extracting shared logic into `_find_matching_unfinished_file`. This
  increased code reuse, and improved code maintenance and readability.

- Refactoring `_find_unfinished_file_by_plan_id` and
  `_match_unfinished_file_if_possible` to use the shared logic, enhancing
  their readability and reliability.

- Enhancing function documentation. Docstrings were updated to be more
  informative and adhere to a standard template.

- Adding a note indicating 'listFiles' access is required for the
  operations.