fix: unambiguous get_by_child, deterministic gated-executor race test, exact-line probe exemption

- get_by_child raises ValueError for unknown children: None means ONLY
  'existing child, no schedule' (present every day), never 'nonexistent
  child silently present'
- concurrent-upsert test uses a gated async executor that pauses task A's
  first transactional job after the connection lock is held, then
  schedules task B so it provably queues on the lock; deterministic,
  not timing-dependent
- leak guard's probe exemption narrowed from whole-function spans to the
  exact string-literal lines holding the two sanctioned COUNT(*) queries

custom_components/nestquest/dao_presence.py

@@ -134,9 +134,17 @@ class PresenceSchedulesDao:
     ) -> PresenceScheduleRecord | None:
         """Return the child's schedule, or None when the child has none.
 
-        None is meaningful: a child with no schedule row is present
-        every day (Feature 05 guardrail).
+        None is meaningful and UNAMBIGUOUS here: the child exists and
+        has no schedule row, i.e. the child is present every day
+        (Feature 05 guardrail).  An unknown child id raises ValueError
+        instead of returning None, because "nonexistent child" must
+        never be silently read as "present every day".
         """
+        child = await self._database.fetch_one(
+            "SELECT 1 FROM children WHERE id = ?", (child_id,)
+        )
+        if child is None:
+            raise ValueError(f"child {child_id} does not exist")
         row = await self._database.fetch_one(
             f"SELECT {_SCHEDULE_COLUMNS} FROM presence_schedules "
             "WHERE child_id = ?",