Snippets access rights checking single file constraint

What does this MR do?

Related to #205628

Validate git operations to snippet repositories, ensuring the end result is one single file only.

It will be invalid if repository is in the state of multiple files or no file at all.

Refactor: renames exec method to the common validate! method, which is the correct name for subclasses of BaseChecker.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.