test: Add regression test for Issue #550 (long message overflow)

[securesigner]

This PR addresses Issue #550: "TextDoesNotFitException when trying to sign very long messages".

It adds a regression test (tests/test_issue_550.py) that simulates passing a very long, uninterrupted string (such as a BOLT12 invoice) to the SeedSignMessageConfirmMessageScreen. This test ensures that the text reflow logic handles the overflow gracefully (using allow_text_overflow=True) instead of raising a TextDoesNotFitException and crashing the app.

The fix was confirmed to already be present in the codebase; this PR adds the test case to prevent future regressions.

Screenshots omitted as this PR only adds a backend test case and does not modify the visible UI.

This pull request is categorized as a:

• New feature
• Bug fix
• Code refactor
• Documentation
• Other (Test coverage / Regression test)

Checklist

• I’ve run pytest and made sure all unit tests pass before submitting the PR

If you modified or added functionality/workflow, did you add new unit tests?

• No, I’m a fool
• Yes
• N/A

I have tested this PR on the following platforms/os:

• Raspberry Pi OS [Manual Build]
• [SeedSigner OS]on a Pi0/Pi0W board
• Other (GitHub Codespaces / Linux environment)

[kdmukai]

I think these sorts of issues will be rendered moot by #761

But IF that PR isn't merged, THEN this regression test might be good to have.

[securesigner]

Thanks for the context. I see that #761 is still open and has conflicts, so I'll leave this regression test here as a safety net for the current codebase until that refactor lands. If #761 merges first, I'm happy to update or close this.

[kdmukai]

Yep, that totally makes sense. And good catch on my PR needing an update. I'll see if I can get that updated soon.

[kdmukai]

I've updated PR #761 to resolve the merge conflicts.