fix: Adjusting testing for GCS bucket existence check

[yhakbar]

Description

Adjusting mocking implementation to avoid global anonymous function in GCS bucket existence check, as discussed in #3756.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Updated GCS testing approach.

Migration Guide