Preserve plugin install failure subtypes during imports (#33182)

## What changed

- Add `subErrorType` to external agent config import failures and carry it through progress and completion notifications, persisted import histories, and analytics.
- Expose plugin store failure subtypes from `PluginInstallError` so manual installs and imported plugins report details such as `failed_to_copy_plugin_file` while failures without a subtype remain `null`.
- Keep existing persisted import records compatible by defaulting the new field when it is absent.

## Testing

- Cover store I/O subtype extraction, analytics serialization, persisted import histories, and migration notification fixtures.

GitOrigin-RevId: ec1a85623fc826cbd7cfe9f55c2dd2bc86665937

codex-rs/tui/src/external_agent_config_migration_flow_tests.rs

@@ -123,6 +123,7 @@ fn completed_notification() -> ExternalAgentConfigImportCompletedNotification {
                 failures: vec![ExternalAgentConfigImportItemTypeFailure {
                     item_type: ExternalAgentConfigMigrationItemType::Plugins,
                     error_type: Some("plugin_install_failed".to_string()),
+                    sub_error_type: Some("failed_to_copy_plugin_file".to_string()),
                     failure_stage: "plugin_import".to_string(),
                     message: "install failed".to_string(),
                     cwd: Some(PathBuf::from("/workspace/project")),