fix(core): import LanguageContextSelector in Nlsconfig

[TDannhauer]

Summary

• Import Horde\Core\LanguageContextSelector in Nlsconfig so the injector can create LanguageContext.
• Read the Accept-Language header from the promoted constructor property.
• Construct Nlsconfig in the unit test with the current argument order.

Motivation

Nlsconfig is in namespace Horde\Core\Registry and implements LanguageContextSelector without importing it. PHP therefore looked for Horde\Core\Registry\LanguageContextSelector, which does not exist. Date formatting requests LanguageContext, and the injector failed with Interface "Horde\Core\Registry\LanguageContextSelector" not found.

The same method used a bare $serverEnvironment, so the Accept-Language fallback never ran.

Changes

• Add use Horde\Core\LanguageContextSelector in src/Registry/Nlsconfig.php.
• Use $this->serverEnvironment in preferredLang().
• Pass BackendConfigLoader, then the optional environment and session, in NlsconfigTest::makeNlsconfig().

Test plan

• vendor/bin/phpunit --bootstrap vendor/autoload.php vendor/horde/core/test/Unit/Registry/NlsconfigTest.php (7 tests, 10 assertions)
• Reload Horde and confirm the Cannot create Horde\Core\LanguageContext emergency log lines stop