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/analytics/src/events.rs

@@ -1004,6 +1004,7 @@ pub(crate) struct CodexPluginInstallFailedMetadata {
     pub(crate) plugin: CodexPluginMetadata,
     pub(crate) source: crate::facts::PluginInstallSource,
     pub(crate) error_type: String,
+    pub(crate) sub_error_type: Option<String>,
 }
 
 #[derive(Serialize)]
@@ -1037,6 +1038,7 @@ pub(crate) struct CodexOnboardingExternalAgentImportFailureMetadata {
     pub(crate) item_type: String,
     pub(crate) failure_stage: String,
     pub(crate) error_type: String,
+    pub(crate) sub_error_type: Option<String>,
     pub(crate) product_client_id: Option<String>,
 }