test(gateway): guard every /stop path against session suspension

[r266-tech]

Summary

/stop intentionally interrupts work without suspending the conversation. This
rebases the regression guard onto current main and covers every current route:

• existing sentinel, active-agent, and pending-message tests now assert that
  suspend_session() was not called;
• a no-active-agent test enters through the production message dispatcher and
  checks that the current session remains visible;
• both authorized and unauthorized sibling-thread /stop paths assert session
  preservation.

The patch reuses the existing scenario setup instead of keeping the original
aggregate test. Related PR #9457 overlaps the three older assertions, but its
current head includes unrelated changes and does not cover the no-active or
sibling-thread paths requested in the maintainer review.

Test plan

• uv run pytest -q tests/gateway/test_session_race_guard.py tests/gateway/test_stop_thread_sibling.py — 27 passed
• uv run ruff check tests/gateway/test_session_race_guard.py tests/gateway/test_stop_thread_sibling.py

Follow-up to #9241.

[alt-glitch]

This was generated by AI during triage.

Updated after the live rework: related to #9457, not a duplicate. This branch additionally covers the no-active-agent and authorized/unauthorized sibling-thread /stop routes; maintainer review can choose whether to consolidate the complementary regression coverage.

[teknium1]

Thanks for adding a direct regression guard for the session-preservation behavior introduced by 952a885.

Problems

• Current main has an additional authorized sibling-thread /stop path at gateway/slash_commands.py:1089-1110; the proposed three-path test does not exercise it.
• The sentinel and active-agent scenarios are already exercised in tests/gateway/test_session_race_guard.py:409-520. The missing piece is specifically the suspend_session() assertion.

Suggested changes

• Add runner.session_store.suspend_session.assert_not_called() to the existing sentinel, active-agent, and pending-message tests, as open PR test(gateway): guard /stop against session suspension regression #9457 does, rather than duplicating their setup in one aggregate test.
• Add sibling-thread coverage if this test is intended to guard every current /stop path.

Automated hermes-sweeper review.

[r266-tech]

Reworked this onto current main following the maintainer review:

• moved the preservation assertions into the existing sentinel, active-agent,
  and pending-message tests;
• added the missing no-active route through _handle_message();
• covered both authorized and unauthorized sibling-thread /stop branches.

The branch is now a focused two-test-file diff (31 additions, one deletion).
Validation: 27 targeted tests and Ruff pass; the exact reworked diff received a
Codex adversarial-review verdict of approve.

[alt-glitch]

This was generated by AI during triage.

Related to #9457, not a duplicate after the rework: this branch additionally covers the no-active-agent and authorized/unauthorized sibling-thread /stop routes. Maintainer review can choose whether to consolidate the complementary regression coverage.