Stdlib: sprintf/vsprintf %'<char> custom pad (#22833)

[PurHur]

Summary

• Parse php-src formatted_print.c %'<char> custom pad in VmSprintf so sprintf/vsprintf/printf match Zend instead of LogicException on %'
• Covers pad char, left-adjust + custom pad, positional %1$'*10s, and ValueError: Missing padding character
• php-src ref: ext/standard/formatted_print.c (modifier loop for ')

Closes #22833

Test plan

• php test/repro/issue22833_sprintf_custom_pad.php vs php bin/vm.php — identical
• php bin/jit.php test/repro/issue22833_sprintf_custom_pad.php — matches Zend
• vendor/bin/phpunit --filter 'testVmSprintfCustomPad|sprintf_custom_pad' — OK
• Standalone AOT NestedJIT SprintfJitHelper still only covers %03d/%d/%.*s shapes (pre-existing); custom pad remains VM/Internal path for MCJIT. Follow-up if NestedJIT pad loops are fixed.

Verification transcript

$ php bin/vm.php test/repro/issue22833_sprintf_custom_pad.php
*******************x
---------x
*********7
*******x
*********x
######ab
********+7
********+7
*******7
x*********
ValueError:Missing padding character

$ ./script/docker-exec.sh -- bash -lc 'php bin/jit.php test/repro/issue22833_sprintf_custom_pad_aot.php'
*******************x
*********7
*******x
*********x
x*********

Gate probe: bootstrap-inventory --check → OK 6512/6512. north-star5-fast still red at 4f-m gen-0 (#22642) — unrelated.

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