Stdlib: DateTime accepts Y-m-d H:i without seconds (#25309)

[PurHur]

Summary

• Relax VmDateTimeNative::parseDateTimeAbsolute() ISO regex so Y-m-d H:i / Y-m-dTH:i parse with seconds defaulting to 0 (timelib / php-src parse_date.re).
• Covers DateTime, DateTimeImmutable, and date_create().

Closes #25309

Test plan

• Zend/VM/JIT bit-identical on test/repro/maintainer_gap_datetime_hour_minute.php
• vendor/bin/phpunit --filter datetime_hour_minute_no_seconds — OK (2 tests)
• AOT new DateTime(...) segfaults on master for this surface (pre-existing)

php-src reference

• ext/date/lib/parse_date.re — absolute Y-M-D + optional time with optional seconds

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