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/config.rs

@@ -722,6 +722,10 @@ pub struct ExternalAgentConfigImportParams {
     /// Optional identifier for the product that initiated the import.
     #[ts(optional = nullable)]
     pub source: Option<String>,
+    /// Opaque provider identifier supplied by the caller for analytics attribution. This does not
+    /// select the migration source.
+    #[ts(optional = nullable)]
+    pub provider_id: Option<String>,
     /// Migration-source selector used to produce the migration items. Pass the same value to
     /// detection and import; missing or unrecognized values use the default source.
     #[ts(optional = nullable)]

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,
         }
     );