🧪 Add test for invalid frequency unit in DataFlow

[mysticBliss]

🎯 What: The testing gap addressed

• Added a test case for DataFlow when source_frequency_unit is invalid.

📊 Coverage: What scenarios are now tested

• Validates that ValueError is raised when source_frequency_unit is not 'days' or 'months'.
• Specifically targets the else branch in _generate_dates.

✨ Result: The improvement in test coverage

• Increased coverage for DataFlow.py.
• Ensures that invalid configurations are caught early with expected exceptions.

---

PR created automatically by Jules for task 13132168519546185207 started by @mysticBliss

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.