Handle entry not found in changeFileContent action

• Step 1 - Handle entry not found in changeFileContent action
• Step 2 - Disallow fs in integration specs
• Step 3 - Resolve "Improve slow tests in ./spec/features/ide/user_sees_editor_info_spec.rb"

What does this MR do?

This MR fixes this bug #284930 (closed) by gracefully handling the entry not found case in the changeFileContent action.

Screenshots (strongly suggested)

Still works and no console errors after rename 😄

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

Related to #284930 (closed)