[php-src] PR #24723: Use the default depth in the JSON fuzzer and check json_validate()

Pull Request: https://github.com/php/php-src/pull/24723 Author: yumasansansan - php_json_parser_init(..., 10) has been in the fuzzer since it was added in 2019, with no stated reason. json_decode() and json_validate() default to PHP_JSON_PARSER_DEFAULT_DEPTH (512). The fuzzer now uses that. - json_validate() (php_json_validate_ex()) runs the same scanner and grammar with a separate set of parser methods that build no values. It was not covered by any fuzzer. For JSON_OBJECT_AS_ARRAY (with and without JSON_INVALID_UTF8_IGNORE), decoding fails exactly when validation fails, apart from running out of memory, which cannot happen in this fuzzer. Decoding to objects is excluded because it additionally rejects property names starting with a NUL byte. The fuzzer asserts that both agree. Replaying the checked-in sapi/fuzzer/corpus/json (no fuzzing): edge coverage 1211 → 1257, and the assertion holds for every seed. The input format is unchanged.