Fix base_convert() enum case operands — TypeError parity (#8976)

[PurHur]

Summary

• Route base_convert() $num through VmString::coerceStringBuiltinArg (VM) and JitStringBuiltinArg::lower (JIT/AOT) so backed enum cases throw TypeError like Zend Z_PARAM_STR, instead of LogicException or silent backing coercion.
• php-src reference: ext/standard/string.c — PHP_FUNCTION(base_convert)

Verification

# Issue repro — VM matches Zend TypeError
php test/repro-maintainer/parity_base_convert_enum.php
php bin/vm.php test/repro-maintainer/parity_base_convert_enum.php
# TypeError: base_convert(): Argument #1 ($num) must be of type string, Es given

# Scalar unchanged
php bin/vm.php -r 'echo base_convert("ff", 16, 10), "\n";'
# 255

# JIT
php bin/jit.php -r 'enum Es: string { case A = "ff"; } try { base_convert(Es::A,16,10); } catch (TypeError $e) { echo $e->getMessage(), "\n"; }'
# base_convert(): Argument #1 ($num) must be of type string, Es given

# Compliance (Docker)
./script/docker-exec.sh -- bash -lc 'php vendor/bin/phpunit test/compliance/VMTest.php --filter base_convert_enum_typeerror'
# OK (2 tests)

# AOT compile-only
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/compile.php -l test/fixtures/aot/compile-only/base_convert_enum_typeerror.php'
# exit 0

php vendor/bin/phpunit test/unit/AotRegressionFixtureCatalogTest.php --filter base_convert_enum
# OK (1 test)

Closes #8976

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