Refactor rich_content_editor mocks (tighter cohesion)

What does this MR do?

It improves the cohesion of mocks for the rich_content_editor.vue as the prior approach had the mock_data.js hold shared responsibility for what otherwise should remain as close to the usage as possible.

Screenshots

n/a

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

Closes #224679 (closed)