Add model validation to Design to limit filename

What does this MR do?

This MR adds a new validation to the Design model to limit the filename to 255 characters.

There is a corresponding migration MR !33565 (merged).

• Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/209025

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.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done