Attribute external agent imports by provider (#34451)

## What changed

- Add an optional opaque `providerId` to `externalAgentConfig/import` for analytics attribution, independently of the `migrationSource` selector.
- Include the provider ID in completed-import and import-failure analytics events.
- Have TUI imports identify their selected migration source as the provider.

## Testing

- Cover provider attribution in app-server import completion and failure tests and analytics serialization tests.

GitOrigin-RevId: 0ee7313a43652e5305939c526bf6b903a79334ad

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -436,6 +436,7 @@ fn external_agent_config_import_params_accept_legacy_plugin_details() {
                 }),
             }],
             source: None,
+            provider_id: None,
             migration_source: None,
         }
     );