Stdlib: highlight_*()/show_source() enum operands TypeError (#6486)

[PurHur]

Summary

• Replace manual TYPE_STRING checks (which threw uncatchable LogicException) with VmString::coerceStringBuiltinArg() on highlight_string(), highlight_file(), and show_source() so backed enum case operands raise catchable TypeError matching php-src ext/standard/url.c (Z_PARAM_STR).
• Extract highlight_file::run() so the show_source alias reports the correct function name in error messages.
• Add maintainer repro + compliance PHPT.

php-src reference: ext/standard/url.c — PHP_FUNCTION(highlight_string), PHP_FUNCTION(highlight_file)

PHP implementation: ext/standard/highlight_string.php, highlight_file.php, show_source.php

Closes #6486

Verification

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro-maintainer/issue_highlight_enum_typeerror.php'

Output:

highlight_string: TypeError: highlight_string(): Argument #1 ($string) must be of type string, E given
highlight_file: TypeError: highlight_file(): Argument #1 ($filename) must be of type string, E given
show_source: TypeError: show_source(): Argument #1 ($filename) must be of type string, E given

./script/docker-exec.sh -- bash -lc './vendor/bin/phpunit --filter highlight_enum test/compliance/'

OK (1 test, 1 assertion)

./script/docker-exec.sh -- bash -lc './vendor/bin/phpunit --filter "highlight_string|highlight_enum" test/compliance/'

OK, but incomplete, skipped, or risky tests!
Tests: 4, Assertions: 4, Skipped: 1.

Note: highlight builtins remain VM-only per existing capability matrix (#3164); no AOT/JIT lowering change in this PR.

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