chore: improve validators maintenance path

[lphuc2250gma]

Summary:

• Add edge-case unit tests for the IBAN validator: whitespace-only input, lowercase-letter normalization round-trip, and a small set of known-invalid country-code prefixes — pure additive tests in tests/test_iban.py, no behavior change.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.