fix #3114: prevent scientific notation when generating bank account number

[AhmedHisham552]

Motivation / Background

This Pull Request has been created to fix issue #3114, a bug in Faker::Bank.account_number where generated values could sometimes include scientific notation (e.g., 3512553848424886e-05) due to using rand.to_s[2..].

This resulted in invalid bank account numbers containing non-digit characters.

The new implementation ensures that account numbers always consist of digits only.

Additional information

• The fix uses integer-based digit generation, avoiding reliance on stringified floats.

Checklist

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• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
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• Tests are added or updated if you fix a bug, refactor something, or add a feature.
• Tests and Rubocop are passing before submitting your proposed changes.

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• Double-check the existing generators documentation to make sure the new generator you want to add doesn't already exist.
• You've reviewed and followed the Contributing guidelines.

[jremes-foss]

LGTM ✅

[thdaraujo]

thanks!