[codex] Fix silent partial-stream recovery and bounded gateway restart

[0-CYBERDYNE-SYSTEMS-0]

Summary

Fixes two related gateway failure modes observed during long Telegram agent runs:

• partial stream recovery now surfaces a visible abnormal-turn explanation instead of treating the recovered fragment as a normal final preview
• detached in-chat /restart now launches its external restart helper before graceful drain and bounds the helper wait by restart_drain_timeout + 5s

Root Cause

When streaming failed after partial delivery, partial_stream_recovery reused the streamed fragment as the final response and marked response_previewed=True. The gateway could then suppress normal final delivery even though the user only saw a partial stream, leaving the failure silent.

For /restart, the detached restart helper was previously launched from the shutdown path after drain work. If the gateway accepted /restart but then wedged during graceful drain, the helper could be delayed or wait indefinitely for the stuck PID to exit.

Changes

• Keep response_previewed=False for partial stream recovery so gateway fallback delivery can send the recovered text plus the explanation.
• Always append the turn-completion explainer for partial_stream_recovery, not only for empty or very short fragments.
• Launch the detached restart helper as soon as a detached restart is requested.
• Add an idempotency guard for the helper and bound its PID wait by the configured drain timeout plus a small grace window.
• Add regression coverage for partial stream recovery visibility and detached restart helper behavior.

Validation

.venv/bin/python -m py_compile agent/conversation_loop.py run_agent.py tests/run_agent/test_run_agent.py tests/run_agent/test_turn_completion_explainer.py gateway/run.py tests/gateway/test_restart_drain.py
.venv/bin/pytest tests/run_agent/test_turn_completion_explainer.py tests/run_agent/test_run_agent.py::TestRunConversation::test_partial_stream_recovery_on_empty_stub tests/run_agent/test_run_agent.py::TestRunConversation::test_partial_stream_recovery_preempts_prior_turn_fallback tests/gateway/test_restart_drain.py -q
scripts/run_tests.sh tests/run_agent/test_turn_completion_explainer.py tests/gateway/test_restart_drain.py tests/run_agent/test_run_agent.py -- -q

32 passed for the focused pytest run, and 397 tests passed through scripts/run_tests.sh for the affected files.

Scope Note

This fixes silent partial-stream presentation and the accepted /restart drain path. A gateway process that is already unable to process Telegram updates still needs a separate external watchdog or persistence-timeout follow-up.

[teknium1]

Merged via #53987. Your partial-stream-recovery fix was reworked onto current main with authorship preserved in git log (commit e860a40). The recovered fragment now stays unpreviewed so the gateway delivers it plus a visible 'send continue' footer. Thanks for the original fix!