Add ECA execute action for Rerank
### Problem/Motivation
The AI module's `Rerank` operation type has no corresponding ECA execute action in this module. ECA flows cannot drive `Rerank` without a custom action plugin.
Existing per-operation actions: Chat, Embedding, Moderation, Speech to Text, Text to Speech (`src/Plugin/Action/`).
### Proposed resolution
Add `Plugin\Action\Rerank` extending `AiConfigActionBase`, mirroring `Chat.php`:
- Plugin id: `ai_integration_eca_execute_rerank`
- Builds the matching Input class from `Drupal\ai\OperationType\Rerank\` and dispatches it through the configured provider.
- Add a `Kernel\Plugin\Action\RerankTest` mirroring the existing per-action tests.
### API changes
None.
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_Filed with AI assistance (Claude Code)._
issue