mbstring: mb_language() / mb_http_input() encoding metadata (#4636)

[PurHur]

Summary

• Register mb_language() and mb_http_input() in ext/mbstring/ with PHP-in-PHP state in MbstringState (completes Stdlib: mb_internal_encoding() / mb_language() / mb_http_input() — encoding defaults (ext/mbstring parity) #4636 encoding-metadata slice alongside existing mb_internal_encoding()).
• MbstringLanguageRegistry maps php-src NLS language names/aliases; invalid language/type operands throw ValueError matching Zend 8.2.
• Spine + capability matrix updated for the two new builtins.

php-src reference

• ext/mbstring/mbstring.c — PHP_FUNCTION(mb_language), PHP_FUNCTION(mb_http_input)

PHP implementation

• ext/mbstring/mb_language.php, mb_http_input.php
• ext/mbstring/MbstringLanguageRegistry.php, MbstringState.php

Verification

source script/php-env.sh
php bin/vm.php test/repro/maintainer_gap_mb_language_http_input.php
# ok

./vendor/bin/phpunit --filter mb_language_http_input
# OK (2 tests, VM+JIT)

php script/check-selfhost-spine-coverage-sync.php
# OK (4414/4414)

./script/ci-fast.sh
# exit 0

Closes #4636

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