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/schema/json/v2/ExternalAgentConfigImportProgressNotification.json

@@ -29,6 +29,12 @@
             "string",
             "null"
           ]
+        },
+        "subErrorType": {
+          "type": [
+            "string",
+            "null"
+          ]
         }
       },
       "required": [