Stdlib: utf8_decode() scalar coercion + TypeError parity (#4317)

[PurHur]

Summary

• utf8_decode() VM path now uses VmString::coerceStringBuiltinArg() (php-src convert_to_string on arg Php stdlib specs implementation #1) instead of LogicException for non-strings
• JIT/AOT path uses JitStringBuiltinArg::lower() matching utf8_encode()
• Align utf8_encode() TypeError param name to $string per php-src stub
• Add compliance + AOT fixtures; maintainer repro at test/repro-maintainer/utf8_encode_scalar.php

php-src reference: ext/standard/basic_functions.c — PHP_FUNCTION(utf8_encode), PHP_FUNCTION(utf8_decode)

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro-maintainer/utf8_encode_scalar.php
php bin/vm.php test/repro-maintainer/utf8_encode_scalar.php
php bin/jit.php test/repro-maintainer/utf8_encode_scalar.php
'

Output:

'65'
'195'
encode TypeError
decode TypeError
(same on VM and JIT)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter utf8_encode_decode_scalar'

OK (4 tests, 8 assertions)

Closes #4317

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