fix(dispatch): stop burning breaker strikes on dependency-held blocked tasks; collision notify rides the delegate transaction (#726)

* fix(dispatch): stop burning breaker strikes on dependency-held blocked tasks; collision notify rides the delegate transaction

Two follow-ups from the 2026-07-29 blocked-task wave:

- _dispatch_blocker_work now skips a blocked task whose dependencies are
  non-terminal BEFORE the respawn gate. spawn_agent's readiness gate was
  refusing these anyway, but each refused attempt burned a respawn-
  breaker strike and, once tripped at 4, a duplicate CEO escalation
  every tick (seen live on the eslint-audit task held behind a paused
  backend dependency). The dispatcher re-checks each tick and proceeds
  the moment the dependency goes terminal — same resume path as before,
  minus the noise.

- send_collision_sequencing_notification gains the db_session threading
  its sibling senders already have, and TaskService passes its own
  session. delegate wires collision edges inside a transaction whose
  held-back child row is not yet committed; the notification INSERT ran
  on a separate auto-commit connection and died on the related_task_id
  FK (ForeignKeyViolationError seen live in cell_pm/delegate), silently
  losing the coordination alert. Riding the caller's transaction makes
  the row visible and commits both atomically.

* test(notification): cast the fake session for the gate's tests-scope mypy

---------

Co-authored-by: Renn F <rennf93@users.noreply.github.com>

tests/unit/runtime/test_blocker_and_claimed_dispatch.py

@@ -156,6 +156,40 @@ async def test_dispatch_blocker_work_spawns_non_hitl_blocked_task(
     spawn.assert_awaited_once()
 
 
+@pytest.mark.asyncio
+async def test_dispatch_blocker_work_skips_dependency_held_task(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """2026-07-29: a blocked task waiting on a non-terminal dependency must
+    be skipped BEFORE the respawn gate — spawn_agent's readiness gate was
+    going to refuse anyway, and each refused attempt burned a breaker strike
+    plus, once tripped, a CEO escalation notification."""
+    orch = _orch()
+    task: dict[str, Any] = {
+        "id": "t1",
+        "status": "blocked",
+        "blocker_resolver_type": None,
+        "team": "backend",
+        "assigned_to": AGENT_UUIDS["be-pm"],
+        "dependency_ids": ["d1"],
+    }
+    monkeypatch.setattr(orch, "_fetch_tasks", AsyncMock(return_value=[task]))
+    monkeypatch.setattr(
+        orch,
+        "_check_dependencies_terminal",
+        AsyncMock(return_value="Task t1 waiting on non-terminal dependency d1"),
+    )
+    gate = AsyncMock(return_value=False)
+    monkeypatch.setattr(orch, "_pm_respawn_should_gate", gate)
+    spawn = AsyncMock()
+    monkeypatch.setattr(orch, "spawn_agent", spawn)
+
+    await orch._dispatch_blocker_work(client=MagicMock())
+
+    spawn.assert_not_awaited()
+    gate.assert_not_awaited()
+
+
 # ---------------------------------------------------------------------------
 # _claimed_task_needs_agent — claimed-but-no-agent detection
 # ---------------------------------------------------------------------------