FEATURE: persona/tool import and export

[SamSaffron]

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.