[Yaml] Throw a ParseException on keys starting with a NUL byte when parsing mappings as objects - #66830

With Yaml::PARSE_OBJECT_FOR_MAP, a mapping key that starts with a NUL byte crashes the block parser with a PHP \Error, because PHP does not allow such property names on stdClass. Code that catches ParseException does not catch it.

Yaml::parse("'\0': 1\n", Yaml::PARSE_OBJECT_FOR_MAP);
// Before: Error: Cannot access property starting with "\0"
// After: ParseException: Mapping keys starting with a NUL byte are not supported with the "Yaml::PARSE_OBJECT_FOR_MAP" flag at line 1 (near "'\0': 1").

The inline parser does not crash, because it uses an (object) cast. But the cast gives a broken object: { "\0": 1 } gets an inaccessible property, and { "\0*\0x": 1 } gets a protected property named x. So both parsers now throw the same ParseException when they read such a key with the flag. Keys with a NUL byte in other positions ("a\0b") are valid property names and still work. Without the flag, nothing changes.

Tests: ParserTest::testKeyStartingWithNulByteThrowsParseExceptionWhenMappingsAreParsedAsObjects and InlineTest::testParseKeyStartingWithNulByteWithMapObjectsThrowsParseException fail without the fix. The Yaml test suite passes on 6.4 (934 tests). I also cherry-picked the commit onto 7.4, 8.1 and 8.2 without conflicts, and the Yaml suite passes on 8.2.