Standardize @useCodeMirror usage in FileDiffCard components

[coderabbitai]

The following instances of FileDiffCard do not specify the @useCodeMirror property:

• app/components/course-page/test-results-bar/autofix-section/autofix-result.hbs
• app/components/course-page/course-stage-step/second-stage-instructions-card/implement-solution-step.hbs
• app/components/course-page/course-stage-step/first-stage-tutorial-card/uncomment-code-step.hbs

To ensure consistent rendering behavior, we should update these instances to include the @useCodeMirror property.

Referenced from PR #2408: #2408
Comment URL: #2408 (comment)

cc @VasylMarchuk