Simplify structured output configuration in the Chat action
### Problem/Motivation
The Chat action exposes the structured-output JSON schema as a plain text field in its config form (`src/Plugin/Action/Chat.php:46–50`) and stores it as a raw JSON string that is `json_decode()`d at runtime (line 115). That makes it cumbersome to **add** (no validation, no editor affordances, easy to ship malformed JSON) and hard to **look at** once saved — it's a wall of escaped JSON in the form.
### Proposed resolution
Investigate replacing the raw text input with the AI module's dedicated `'#type' => 'ai_json_schema'` form element (`Drupal\ai\Element\AiJsonSchema`), so:
- **Adding** a schema uses a purpose-built editor (validation, structure-aware UI) instead of free-form text.
- **Looking at** a saved schema renders as a readable, possibly collapsible, structured preview rather than escaped JSON.
Apply the same treatment to any other action / submodule that takes a structured-output schema (to be scoped during the work). The stored value can stay as a JSON string for backward compatibility.
### API changes
None expected.
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_Filed with AI assistance (Claude Code)._
issue