🧪 [test] Add test coverage for invalid USERNAME validation in Mermaid feature

[MiguelRodo]

🎯 What: Adds a standalone test to cover the validation gap in src/mermaid/install.sh where the script ensures USERNAME is formatted correctly (preventing option/command injection).
📊 Coverage: The test runs the installation script locally with USERNAME="-invalid" and asserts that the failure condition is triggered and the correct error message is logged.
✨ Result: Enhanced test coverage that catches bugs in user input validation logic before they cause silent or confusing failures during the DevContainer build phase.

---

PR created automatically by Jules for task 3200687924583980853 started by @MiguelRodo

[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.