Stdlib: preg_split/preg_grep mask unknown flags like Zend (#27946)

[PurHur]

Summary

• Remove over-strict LogicException on unknown preg_split / preg_grep flags; match php-src bit-mask behavior.
• preg_grep: invert via flags & PREG_GREP_INVERT (VM + JIT).
• preg_split: PREG_SPLIT_DELIM_CAPTURE only emits capturing subgroups (never full match when pattern has no ()), so flags like 999 match Zend [].
• Keep preg_match bad-flags ValueError.

Closes #27946

Test plan

• Zend / VM / JIT on test/repro/issue_27946_preg_split_grep_unknown_flags.php — identical
• vendor/bin/phpunit --filter PregSplitBuiltinTest — OK
• Compliance .phpt + unit guard

Verification

split:[]
grep:[1]
grep998:{"1":"a"}
match:ValueError:preg_match(): Argument #4 ($flags) must be a PREG_* constant

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