fix(container): emit SSE done event on stream end

[mariuspruvot]

Summary

• The SSE stream endpoint now emits an explicit event: done frame when the container exits (docker logs end)
• Also calls mark_completed() to transition the session to COMPLETED in the DB
• Previously the frontend had no reliable signal that the session was over -- with the per-turn entrypoint loop (PR fix(container): multi-turn FIFO persistence #16), the container stays alive between turns, so the SSE connection no longer closes after the first response

Changes

File	What
apps/api/src/helprs/modules/container/router.py	_event_stream() yields event: done + calls mark_completed() after stream ends
apps/api/tests/modules/container/test_router.py	2 new tests: done event emitted + session marked COMPLETED. Also fixed FakeDockerClientForRouter (newline-terminated logs, write_to_container), registered set_session_factory in fixture

Test plan

• 7/7 router tests pass (including 2 new)
• 227/230 full suite pass (3 pre-existing failures unrelated)
• Ruff clean
• Manual e2e: verify frontend receives done event and shows "Session completed"

[helprs-local-mariuspruvot]

helPRs session created for this PR.

Skill: challenge-me | Open session