fix: escape boundary labels in DFD output

[rajanpanth]

Summary

• Escapes boundary names before inserting them into Graphviz HTML-like labels
• Adds a regression test for a boundary name containing >

Closes #278.

Testing

• Direct reproduction check:
  • generated a DFD with Boundary("Client > API")
  • verified output contains Client > API and not the unescaped label
• Not run: full pytest suite is unavailable because pytest is not installed in the current Python environment.

Notes

AI assistance was used to help prepare this change.

[NoodlesNZ]

LGTM, although you need to fix your AI environment if you're going to suggest changes like this. It's trivial to get pytest working, you can run it via poetry easily. I've validated that it passes and when the Github CI kicks off, it will also validate this.