Fix PHPStan drupal.symfonyYamlParse rule failures in ActiveThemeChangeSubscriber

Problem/Motivation

The drupalci PHPStan job fails with 2 errors on the drupal.symfonyYamlParse rule in src/EventSubscriber/ActiveThemeChangeSubscriber.php:

Avoid calling Symfony\Component\Yaml\Yaml::parse() directly. Use \Drupal\Component\Serialization\Yaml::decode() instead, which handles exceptions consistently and applies the correct parse flags.

This is a newly-enforced PHPStan rule, so it now fails any new pipeline on the 4.0.x branch, including pipelines for unrelated changes. In particular it is currently blocking issue #3611182 / MR !46, which does not touch this file at all.

Steps to reproduce

1. Run the project's PHPStan job (drupalci template) against the 4.0.x branch.
2. Observe 2 failures on the drupal.symfonyYamlParse rule, both in src/EventSubscriber/ActiveThemeChangeSubscriber.php (the calls to Yaml::parse() in processConfigWithPatterns() and themeHasFlag()).

Proposed resolution

Replace the two Symfony\Component\Yaml\Yaml::parse() calls in ActiveThemeChangeSubscriber.php with \Drupal\Component\Serialization\Yaml::decode(), and swap the corresponding Symfony\Component\Yaml\Exception\ParseException catch clauses for \Drupal\Component\Serialization\Exception\InvalidDataTypeException, per the Drupal core PHPStan rule. No behavioural change: Yaml::decode() is Drupal core's own wrapper around the Symfony YAML parser.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Update Release Notes
• ❌ Release

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• N/A