test(demo): add unit tests for core formatters

[Ugarba202]

Summary

Add comprehensive unit tests for the formatting utilities in the BDK Demo app to ensure balances, addresses, and timestamps display correctly to the user.

Scope

This PR only adds:

• bdk_demo/test/core/utils/formatters_test.dart

Validation

• confirmed flutter test passes locally for all cases.
• confirmed formatTimestamp is tested dynamically against DateTime.now() to prevent timezone/year flakiness.

[Johnosezele]

This branch still includes older commits that aren’t part of this change. Pls rebase onto latest main (or open a fresh branch from main and cherry pick only the formatter test commit) so the PR only shows the unit test work.

Also, these are mostly happy path checks. A couple of edge cases would make the suite more useful than broader happy path coverage alone.

Going forward, please open an issue first when you can so we can align on the use case before a PR lands. That helps keep scope clear and avoids rework.

[Ugarba202]

This branch still includes older commits that aren’t part of this change. Pls rebase onto latest main (or open a fresh branch from main and cherry pick only the formatter test commit) so the PR only shows the unit test work.

Also, these are mostly happy path checks. A couple of edge cases would make the suite more useful than broader happy path coverage alone.

Going forward, please open an issue first when you can so we can align on the use case before a PR lands. That helps keep scope clear and avoids rework.

Thanks for the review and guidance. I'll get this onto a fresh branch from main so the history is clean, and I'll add some edge-case tests like zero/negative balances and empty strings.

[Ugarba202]

I have force-pushed a rebased branch with a clean history and added all the edge case tests requested.
I have also noted on opening an issue first going, forward i will definitely do that for my next contribution! @Johnosezele

[Johnosezele]

ACK d0e91a2