Follow-up to the salvaged #34452 turn-completion explainer:
- Register display.turn_completion_explainer: True in DEFAULT_CONFIG so the
setting is discoverable, matching the file_mutation_verifier precedent.
- Shorten the repeated footer prefix from 'Turn ended without a usable
reply: ' to 'No reply: ' so the 10 reason variants don't all open with
the same 8-word boilerplate.
- Update the 7 assertions that referenced the old prefix.
When a turn ends abnormally after substantive tool calls (empty content
after retries, a partial/truncated stream, exhausted retries, or an
iteration/budget limit), the CLI/TUI response area was left blank or
showed only a fragment (e.g. "The") with no consolidated reason. The
internal turn_exit_reason values (empty_response_exhausted,
partial_stream_recovery, etc.) were never surfaced to the user.
Add a turn-completion explainer that mirrors the existing file-mutation
verifier footer: at turn end, map an abnormal turn_exit_reason to a
short, actionable message and either replace the bare "(empty)"
sentinel or append the reason after a partial fragment. Normal
text_response exits (e.g. a terse "Done.") stay quiet.
Gated by display.turn_completion_explainer (default on) with
HERMES_TURN_COMPLETION_EXPLAINER env override, matching the
file-mutation verifier seam.
Closes#34452
