Stdlib: json_validate() — validate JSON without decode (#3101)

[PurHur]

Summary

Implements PHP 8.3 json_validate() for VM, JIT, and AOT:

• VM: VmJsonValidate uses host json_validate() when available (PHP 8.3+), otherwise json_decode() + json_last_error() with ValueError on depth exceed
• JIT/AOT: __compiler_json_validate in phpc_json_decode.c reuses the existing JSON parser with configurable max_depth; returns native int1 for correct ternary truthiness
• Registered in ext/standard/Module.php, JIT builtins, self-host policy, and capability matrix

php-src reference

• ext/json/json.c — PHP_FUNCTION(json_validate) / php_json_validate_ex

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && \
  php vendor/bin/phpunit test/unit/JsonValidateBuiltinTest.php && \
  php vendor/bin/phpunit test/compliance/VMTest.php --filter json_validate && \
  php vendor/bin/phpunit test/aot/AotTest.php --filter json_validate'
php bin/vm.php -r 'var_export(json_validate("{\"a\":1}"));'

Closes #3101

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