Verify DAG source codetab functionality

1. Navigate to a DAG's Code tab
2. Verify DAG source code is displayed
3. Verify syntax highlighting is applied
4. Verify code is scrollable for long files
5. Verify line numbers are displayed

Closes : #59546
Related : #59028

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.