Add attribute plugin discovery to ContextReactionManager

Problem/Motivation

On Drupal 11.2+, the Context module triggers the following deprecation:

Not supporting attribute discovery in Drupal\context\ContextReactionManager is deprecated in drupal:11.2.0 and is removed from drupal:12.0.0. Provide an Attribute class and an Annotation class for BC. See https://www.drupal.org/node/3395582

Steps to reproduce

1. Install and enable Context on Drupal 11.2 or later.
2. Run a deprecation check, such as Upgrade Status.
3. Observe the deprecation for Drupal\context\ContextReactionManager.

Proposed resolution

• Add an attribute class for Context reaction plugins, with constructor parameters that map one-to-one to the existing ContextReaction annotation properties.
• Update ContextReactionManager to pass the attribute class first and the existing annotation class second to DefaultPluginManager::__construct(). Retain the annotation class for backwards compatibility.

  Conceptually, the plugin manager constructor call should become:

  parent::__construct(
    'Plugin/ContextReaction',
    $namespaces,
    $module_handler,
    ContextReactionInterface::class,
    \Drupal\context\Attribute\ContextReaction::class,
    \Drupal\context\Annotation\ContextReaction::class,
  );
  

• Convert Context reaction plugin definitions to use the new attribute class.

Remaining tasks

• Add the ContextReaction attribute class.
• Update ContextReactionManager to support attribute discovery with annotation fallback.
• Convert Context reaction plugin definitions to attributes.
• Add or update automated test coverage for plugin discovery.
• Verify that the Drupal 11.2 deprecation is no longer reported.
• Review
• Merge

User interface changes

None.

API changes

A new Drupal\context\Attribute\ContextReaction attribute class will be introduced. The existing annotation class should remain available for backwards compatibility.

Data model changes

None.