Stdlib: mb_strlen() — enum case operand TypeError (#5873)

[PurHur]

Summary

• mb_strlen() VM path now uses VmString::coerceStringBuiltinArg() (php-src Z_PARAM_STR / ext/mbstring/mbstring.c) instead of throwing LogicException on enum case operands.
• JIT/AOT lowering uses JitStringBuiltinArg::lower() via refactored JitMbStrlen::utf8LengthFromPtr().
• Adds VM/JIT compliance PHPTs, AOT fixture, and wires MbStrlenVMTest / MbStrlenJITTest (existing mb_strlen*.phpt were previously unwired).

Closes #5873

php-src reference

• ext/mbstring/mbstring.c — PHP_FUNCTION(mb_strlen) / Z_PARAM_STR
• PHP implementation: ext/mbstring/mb_strlen.php, ext/mbstring/JitMbStrlen.php

Verification

docker info >/dev/null
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro/maintainer_mb_strlen_enum.php
php bin/vm.php test/repro/maintainer_mb_strlen_enum.php'
# Zend: TypeError: mb_strlen(): Argument #1 ($string) must be of type string, Es given
# VM:   TypeError: mb_strlen(): Argument #1 ($string) must be of type string, Es given

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
vendor/bin/phpunit test/compliance/MbStrlenVMTest.php
vendor/bin/phpunit test/compliance/MbStrlenJITTest.php'
# OK (2 tests each)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/compile.php -l test/repro/maintainer_mb_strlen_enum.php'
# exit 0 (AOT lint)

Note: full AOT link (compile.php -o) currently fails on master in this harness with pre-existing StringSerializeDoubleJit LLVM builder error; same failure affects other AOT fixtures (e.g. levenshtein).

Test plan

• VM repro matches Zend TypeError message
• JIT enum TypeError compliance green
• Normal string operands unchanged (mb_strlen.phpt, mb_strlen_jit.phpt)
• AOT lint (compile.php -l) green

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