Report I/O subtypes for session config import failures - #37723

## What changed

- Append a stable `std::io::ErrorKind` category to the
  `failed_to_load_session_config` subtype, including categories such as
  `invalid_data`, `not_found`, and `permission_denied`.
- Propagate the categorized subtype through session import failure reporting.

## Testing

- Add an app-server test that verifies an invalid config reports
  `failed_to_load_session_config_invalid_data` in both the import completion
  notification and analytics event.

GitOrigin-RevId: e9a1a7cd36979911f422805180dd91b6ed65e14e