Add ECA execute action for Speech to Speech
### Problem/Motivation
The AI module's `SpeechToSpeech` operation type has no corresponding ECA execute action in this module. ECA flows cannot drive `SpeechToSpeech` 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\SpeechToSpeech` extending `AiConfigActionBase`, mirroring `Chat.php`:
- Plugin id: `ai_integration_eca_execute_speech_to_speech`
- Builds the matching Input class from `Drupal\ai\OperationType\SpeechToSpeech\` and dispatches it through the configured provider.
- Add a `Kernel\Plugin\Action\SpeechToSpeechTest` mirroring the existing per-action tests.
### API changes
None.
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_Filed with AI assistance (Claude Code)._
issue