Follow-up to the #55780 dead-target not_found blast-radius fix (merged in
#56225). classify_send_error and is_chat_level_not_found each built their own
lowercased error blob, but divergently: classify_send_error appended the
exception CLASS NAME while is_chat_level_not_found did not. A caller passing
exc= to both could get inconsistent answers on the same failure.
- Extract _error_blob(exc, error_text) as the single source of truth both
classifiers use (str(exc) when non-empty + class name; no stray leading
space).
- Align is_chat_level_not_found's signature to (exc, error_text), matching
classify_send_error, removing the swapped-positional footgun; update the
sole caller and the three tests to keyword form.
- Add a regression guard asserting _error_blob keeps the class name.
Surfaced by the hermes-pr-review Phase 2c structured review of #56225.
53b017f03e