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/app-server-protocol/src/protocol/v2/config.rs

@@ -715,6 +715,7 @@ pub struct ExternalAgentConfigImportResponse {
 pub struct ExternalAgentConfigImportItemTypeFailure {
     pub item_type: ExternalAgentConfigMigrationItemType,
     pub error_type: Option<String>,
+    pub sub_error_type: Option<String>,
     pub failure_stage: String,
     pub message: String,
     pub cwd: Option<PathBuf>,