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/core-plugins/src/manager_tests.rs

@@ -5950,3 +5950,16 @@ fn remote_installed_plugins_cache_refresh_coalesces_materializations() {
         0
     );
 }
+
+#[test]
+fn plugin_install_error_preserves_store_io_sub_error_type() {
+    let error = PluginInstallError::Store(PluginStoreError::Io {
+        context: "failed to copy plugin file",
+        source: std::io::Error::other("copy failed"),
+    });
+
+    assert_eq!(
+        error.sub_error_type(),
+        Some("failed_to_copy_plugin_file".to_string())
+    );
+}