Fix suggestion on lines that are not part of an MR

What does this MR do?

Return the text as plain string in the response instead of including HTML tags but keep rich_text as is.

The fix is to modify Blob::UnfoldPresenter#diff_files to map each raw diff line (limited by the range specified) to a corresponding line in an array of highlighted lines to use as rich_text.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

#57953 (closed)