[php-src] PR #25123: Fix GH-23106: mb_strpos() reads past the end of a haystack ending in a truncated UTF-8 sequence

Pull Request: https://github.com/php/php-src/pull/25123 Author: VishnuSrivatsava Fixes the PHP-8.2 part of #25025 and GH-23106. Backport of master commit cc22056f9d (PR #23107, credited to Lazizbek Ergashev). As analyzed in #25025, on this branch the offset walk lives in mbfl_find_offset_utf8() in libmbfl, so the master patch had to be rewritten for the different function and const unsigned char * pointers rather than applied directly. The forward walk can land past the end of the haystack on a truncated multi-byte sequence, and the returned pointer is then used by zend_memnstr() despite violating its end >= p invariant. The clamp restores the invariant for the forward path and for the needle-offset walk feeding zend_memnrstr() on the reverse path. Changes: - Clamp mbfl_find_offset_utf8() to the end of the haystack after the forward offset walk. - Port the regression test from PR #23107 as ext/mbstring/tests/gh23106.phpt. Verification on this branch (macOS arm64, --enable-debug + AddressSanitizer): - Pre-fix: mb_strpos("AA\xf0\x90", "xyz", 3) aborts with Assertion failed: (end >= p), function zend_memnstr, file zend_operators.h, line 179. - Post-fix: assertion gone, regression test passes, full ext/mbstring suite passes (327 passed, 0 failed). Not touching NEWS, following the branch convention where release managers add entries at packaging time. 8.2 is in security-only support until 2026-12-31; since this is an out-of-bounds read, it should qualify for a security release, but that call belongs to release managers. The PHP-8.3 backport is in #25121.