[Php74] Align mb_str_split() polyfill with Mbstring and native

[nicolas-grekas]

Refs #624.

mb_str_split() is polyfilled by both Php74 and Mbstring, and either implementation can end up registered depending on the PHP version and whether ext-mbstring is loaded. The two had drifted:

• Php74::mb_str_split() returned false for a length lower than 1 even on PHP 8+, where the function raises a ValueError.
• It built a single .{N} quantifier, which fails to compile once the length exceeds the PCRE limit of 65535 (preg_split(): Compilation failed: number too big in {} quantifier).

Mbstring::mb_str_split() already handles both. This aligns the Php74 copy with it and with native:

                               native                       Php74 (before)   Php74 (after)
mb_str_split('победа', 0)      ValueError                   false            ValueError
mb_str_split(str_repeat('x',70000), 70000)   [70000 chars]  false (+warning) [70000 chars]

The other overlapping functions (mb_str_pad in Php83, mb_ucfirst/mb_lcfirst/mb_trim/mb_ltrim/mb_rtrim in Php84) were checked against Mbstring and native and already behave identically, so they are left untouched.

Tests exercise Php74::mb_str_split() directly, since the Php74 polyfill is only the active provider of the global function on PHP < 7.4, where the PHP 8 path is unreachable through the global.