Return file name from repo in snippet API endpoints

What does this MR do?

In the snippet entity class, we always return the file_name from the database, but that is no longer valid. Since we don't sync back the content to the db record, the data in the repository is the source of truth.

We must return the file_name from the repository if it exists, otherwise the file_name from the database.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content