fix: keep all digits when formatting 13-digit SP/MG voter ids

[gaoflow]

What this fixes

format_voter_id silently corrupts valid 13-digit voter IDs (the SP/MG edge case, where the sequential number has 9 digits instead of 8).

>>> from brutils import is_valid_voter_id, format_voter_id
>>> is_valid_voter_id("3244567800167")
True
>>> format_voter_id("3244567800167")
'3244 5678 00 16'   # last digit dropped; UF "00"/vd "16" are wrong (real UF=01, vd=67)

is_valid already accepts 13-digit IDs (_is_length_valid allows len == 13 for UF 01/02), and the parsing helpers _get_federative_union / _get_verifying_digits index backwards specifically because "the sequential_number can have eight or nine digits". Only format_voter_id used hardcoded 12-digit forward slices (voter_id[:4], [4:8], [8:10], [10:12]), so it dropped the 13th digit and misplaced the trailing fields.

The change

Make the slicing length-aware, reusing the existing helpers: the federative union and verifying digits are always the last four characters, and the sequential number is everything before them.

• 12-digit IDs format exactly as before (2776 2712 28 52).
• 13-digit IDs now format losslessly (32445 6780 01 67), so the output minus spaces round-trips back to the input.

Tests

Added test_format_voter_id_special_case using the official 13-digit fixture (3244567800167, already asserted valid by test_valid_special_case): it checks validity, the formatted output, and that the result round-trips losslessly. It fails before the change (the last digit is dropped) and passes after. Full suite: 172 tests pass; ruff format --check and ruff check clean.

I used an AI assistant to help investigate this under my direction, and I have reviewed and verified the change myself.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.08%. Comparing base (e1ce34a) to head (d38500b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #742   +/-   ##
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  Coverage   99.08%   99.08%           
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  Files          26       26           
  Lines         765      769    +4     
=======================================
+ Hits          758      762    +4     
  Misses          7        7           

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