fix: bad-character shift in Boyer-Moore search has no effect

[bolverk]

Fix bad-character shift having no effect (dead loop variable)

Fixes #14844

Summary

bad_character_heuristic() in strings/boyer_moore_search.py assigned the
computed shift to the for-loop variable i:

for i in range(self.textLen - self.patLen + 1):
    ...
    i = (mismatch_index - match_index)  # no effect: next iteration overwrites i

In Python, reassigning a for-loop variable inside the loop body has no effect
on the iteration sequence, so the shift was dead code and the search silently ran
as a plain O(n·m) scan over every position.

The rewrite uses a while loop so the shift actually advances the search, and
computes the shift in the pattern's coordinate space (the mismatch offset and
the alignment target are both pattern indices), instead of mixing a text index
with a pattern index as the old code did.

Correctness

The new algorithm was first written in Dafny and machine-verified (15 proof
obligations, 0 errors machine-checked by the SMT-based verifier) to establish:

• Soundness — every emitted position is a genuine occurrence of pattern.
• Completeness — no occurrence is skipped by the shift.

The key safety invariant is documented in the docstring: a jump maps the
mismatching text position onto a pattern index that carries a character unequal
to it, so no skipped alignment can be a match.

In addition to the proof, the implementation agrees with a brute-force scan on
30,000 randomized inputs (empty text, single-character and repeated patterns).

Impact (the issue's own reproduction)

text = "ABCDEFGHIJKLMNOPABCDEFGHIJKLMNOP", pattern = "MNOP":

	iterations	matches
before (brute-force)	29	[12, 28]
after	9	[12, 28]

The existing doctest BoyerMooreSearch("ABAABA", "AB").bad_character_heuristic()
still returns [0, 3].

Checklist

• Fix a bug or typo in an existing algorithm
• I have read CONTRIBUTING.md
• This pull request is all my own work
• This PR only changes one algorithm file
• Passes ruff check, ruff format --check, and the doctests