Introduces import/export feature for tools and personas.
Uploads are omitted for now, and will be added in a future PR
* **Backend:**
* Adds `import` and `export` actions to `Admin::AiPersonasController` and `Admin::AiToolsController`.
* Introduces `DiscourseAi::PersonaExporter` and `DiscourseAi::PersonaImporter` services to manage JSON serialization and deserialization.
* The export format for a persona embeds its associated custom tools. To ensure portability, `AiTool` references are serialized using their `tool_name` rather than their internal database `id`.
* The import logic detects conflicts by name. A `force=true` parameter can be passed to overwrite existing records.
* **Frontend:**
* `AiPersonaListEditor` and `AiToolListEditor` components now include an "Import" button that handles file selection and POSTs the JSON data to the respective `import` endpoint.
* `AiPersonaEditorForm` and `AiToolEditorForm` components feature an "Export" button that triggers a download of the serialized record.
* Handles import conflicts (HTTP `409` for tools, `422` for personas) by showing a `dialog.confirm` prompt to allow the user to force an overwrite.
* **Testing:**
* Adds comprehensive request specs for the new controller actions (`#import`, `#export`).
* Includes unit specs for the `PersonaExporter` and `PersonaImporter` services.
* Persona import and export implemented
