test(widgets): IbanTextFormatter + IbanInputFormatter (+9 tests)

[TaprootFreak]

Summary

Stage 42 of the coverage push. Pure-function tests for the two IBAN formatters that drive the bank-account input field.

Cases

Target	Cases
`IbanTextFormatter.formatIban`	5 — empty; lowercase → upper; strips existing spaces before regrouping; long input groups every four chars; exactly four chars: no trailing space
`IbanInputFormatter.formatEditUpdate`	4 — groups output in fours and uppercases; drops invalid characters silently; caret stays collapsed at the end; empty input remains empty

What's pinned

• `IbanTextFormatter` is the read-side display helper — it does not validate characters, it just regroups and uppercases.
• `IbanInputFormatter` is the write-side `TextInputFormatter`; the regex `[A-Z0-9]` is applied after uppercasing, so lowercase letters survive but punctuation does not. The "drops invalid characters silently" case pins this contract.
• The caret behaviour (`selection: TextSelection.collapsed(offset: text.length)`) is essential UX — explicit case covers it so a future refactor can't silently strand the caret mid-string.

Test plan

• `flutter test test/widgets/iban_formatters_test.dart` — 9 pass
• `flutter analyze` clean on the new file
• CI green