Stdlib: mb_convert_encoding() array|string source operand (#3222)

[PurHur]

Summary

• Implement mb_convert_encoding() array source operand: convert string elements in place, preserve non-string scalars and keys (php-src ext/mbstring/mbstring.c).
• Enum case operands now throw TypeError with Zend array|string expected type (php-src-strict).

php-src reference

• ext/mbstring/mbstring.c — PHP_FUNCTION(mb_convert_encoding) array branch

PHP implementation

• ext/mbstring/VmMbstring.php — convertEncodingSourceArray()
• ext/mbstring/mb_convert_encoding.php — array vs string dispatch

Verification

./script/docker-exec.sh -- bash -lc 'php test/repro/maintainer_gap_mb_convert_encoding_array.php'
./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_mb_convert_encoding_array.php'
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter mb_convert_encoding_array'

ok
ok
OK (2 tests, 2 assertions)

Closes #3222 (array batch slice; iconv/mb_convert_encoding base APIs already registered).

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