feat: Enhance l10n error handling by introducing seedsigner_gettext

[S1DDHEY]

Description

Problem or Issue being addressed

PR #702 (by @AdityaPandeyCN) introduced seedsigner_gettext and seedsigner_ngettext, enhanced gettext wrappers that catch formatting errors in translations (e.g., {} instead of {variable_name}) and raise a descriptive TranslationVariableMissingError.
That PR currently has no test coverage.

Solution

Added comprehensive pytest test coverage for the enhanced gettext functionality in tests/test_l10n_gettext.py (36 tests)

Additional Information

• All tests mock gettext.gettext / gettext.ngettext, no dependency on real .po/.mo files
• Tests mock Settings to avoid hardware/filesystem dependencies
• Manually verified the error handling in the emulator by corrupting the German .po file (replacing {mnemonic_length} and {num_bits} with positional {}), followed by recompiling
• This PR builds on top of [Enhancement] add seedsigner_gettext with improved variable handling  #702
• Addresses issue [Enhancement] Customize gettext with more robust error handling #693

Screenshots

Emulator error screen triggered by intentionally corrupted German translation:

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This pull request is categorized as a:

• New feature
• Bug fix
• Code refactor
• Documentation
• Other

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Checklist

I ran pytest locally

• All tests passed before submitting the PR
• I couldn't run the tests
• N/A

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I included screenshots of any new or modified screens

Should be part of the PR description above.

• Yes
• No
• N/A

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I added or updated tests

Any new or altered functionality should be covered in a unit test. Any new or updated sequences require FlowTests.

• Yes
• No, I’m a fool
• N/A

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I tested this PR hands-on on the following platform(s):

• Raspberry Pi OS Manual Build
• SeedSigner OS on a Pi0/Pi0W board
• Emulator

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I have reviewed these notes:

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• I understand

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