[php-src] PR #25143: Fix GH-25024: backport mb_ereg_replace() \k<name> OOB-read clamp to PHP-8.2/8.3

Pull Request: https://github.com/php/php-src/pull/25143 Author: dhrupo Backports the mb_ereg_replace() \k<name> out-of-bounds read fix (509e27bae1cd, Closes GH-23499; fix by Ilia Alshanetsky) to the still-supported PHP-8.2 and PHP-8.3 branches, as requested in GH-25024. When a \k<name> backref in the replacement string has no closing delimiter, mb_regex_substitute() advances p past eos at two sites and the following smart_str_appendl(pbuf, sp, p - sp) copies the overshoot (a NUL terminator, or heap bytes past the string) into the returned result. Both sites are clamped to eos, identical to PHP-8.4+. Opened against PHP-8.2 so it merges up into 8.3 (8.4 already has the fix). If 8.2 is considered too close to EOL to warrant it, happy to retarget to PHP-8.3. Testing (built from source, NTS, --enable-mbstring): - ext/mbstring/tests/mb_ereg_replace_kname_unterminated_nul.phpt (the upstream test): FAILS on clean 8.2 and 8.3 (output carries a trailing NUL / overshoot), PASSES after the clamps. - Full ext/mbstring/tests: 8.2 = 406 passed / 0 failed (2 skipped), 8.3 = 423 passed / 0 failed (2 skipped). Drafted with an LLM; I reviewed and tested it myself.