GH-50893: [C++][Gandiva] fix out-of-bounds read in soundex mappings lookup

[Arawoof06]

Rationale for this change

soundex_utf8 indexes the 26-entry mappings table with toupper(byte) - 'A', but it classifies letters and folds case with the locale-sensitive isalpha/toupper. Under a non-C locale isalpha accepts bytes 0x80-0xFF and toupper leaves them past 'Z', so the lookup reads past the end of the table. It is reachable from soundex() on untrusted string values; ASAN reports a global-buffer-overflow read for an input byte such as 0xAA when the locale classifies it as a letter.

What changes are included in this PR?

Classify and upper-case with ASCII-only helpers so the table index stays within 0-25. A byte that is not an ASCII letter is treated as a separator, which is what soundex already does for any non-letter.

Are these changes tested?

Yes. TestSoundexNonAsciiNoOverread feeds a value whose non-letter byte is one the active locale reports as a letter; before the change the mappings lookup ran off the end of the table, after it the byte is skipped and the result matches the ASCII-only spelling. The existing soundex cases still pass.

Are there any user-facing changes?

No change for ASCII input. A byte above 0x7f is now consistently ignored instead of being folded through a locale-dependent, out-of-range table lookup.

This PR contains a "Critical Fix". The wrong classification lets soundex() read past the end of the mappings table on an exactly sized input under a non-C locale.

• GitHub Issue: [C++][Gandiva] Out-of-bounds read in soundex_utf8 mappings lookup #50893

[github-actions]

⚠️ GitHub issue #50893 has been automatically assigned in GitHub to PR creator.