fix(state): orphan children instead of cascade-deleting in prune/delete (#6513)

prune_sessions and delete_session only handled direct children when
satisfying the parent_session_id FK constraint. Multi-level chains
(A -> B -> C) caused IntegrityError because deleting B while C still
referenced it was blocked by the FK.

Fix: NULL out parent_session_id for any session whose parent is about
to be deleted. This orphans children instead of cascade-deleting them,
which also respects the prune retention window — newer child sessions
are no longer deleted just because an ancestor is old.

Reported by Aaryan2304 in PR #6463.

tests/test_hermes_state.py

@@ -663,6 +663,84 @@ class TestPruneSessions:
         assert db.get_session("old_cli") is None
         assert db.get_session("old_tg") is not None
 
+    def test_prune_with_multilevel_chain(self, db):
+        """Pruning old sessions orphans newer children instead of crashing on FK."""
+        old_ts = time.time() - 200 * 86400
+        recent_ts = time.time() - 10 * 86400
+
+        # Chain: A (old) -> B (old) -> C (recent) -> D (recent)
+        db.create_session(session_id="A", source="cli")
+        db.end_session("A", end_reason="compressed")
+        db.create_session(session_id="B", source="cli", parent_session_id="A")
+        db.end_session("B", end_reason="compressed")
+        db.create_session(session_id="C", source="cli", parent_session_id="B")
+        db.end_session("C", end_reason="compressed")
+        db.create_session(session_id="D", source="cli", parent_session_id="C")
+        db.end_session("D", end_reason="done")
+
+        # Backdate A and B to be old; C and D stay recent
+        for sid, ts in [("A", old_ts), ("B", old_ts), ("C", recent_ts), ("D", recent_ts)]:
+            db._conn.execute(
+                "UPDATE sessions SET started_at = ? WHERE id = ?", (ts, sid)
+            )
+        db._conn.commit()
+
+        # Should not raise IntegrityError
+        pruned = db.prune_sessions(older_than_days=90)
+        assert pruned == 2  # only A and B
+        assert db.get_session("A") is None
+        assert db.get_session("B") is None
+        # C and D survive, C is orphaned (parent_session_id NULL)
+        c = db.get_session("C")
+        assert c is not None
+        assert c["parent_session_id"] is None
+        d = db.get_session("D")
+        assert d is not None
+        assert d["parent_session_id"] == "C"
+
+    def test_prune_entire_old_chain(self, db):
+        """All sessions in a chain are old — entire chain is pruned."""
+        old_ts = time.time() - 200 * 86400
+
+        db.create_session(session_id="X", source="cli")
+        db.end_session("X", end_reason="compressed")
+        db.create_session(session_id="Y", source="cli", parent_session_id="X")
+        db.end_session("Y", end_reason="compressed")
+        db.create_session(session_id="Z", source="cli", parent_session_id="Y")
+        db.end_session("Z", end_reason="done")
+
+        for sid in ("X", "Y", "Z"):
+            db._conn.execute(
+                "UPDATE sessions SET started_at = ? WHERE id = ?", (old_ts, sid)
+            )
+        db._conn.commit()
+
+        pruned = db.prune_sessions(older_than_days=90)
+        assert pruned == 3
+        for sid in ("X", "Y", "Z"):
+            assert db.get_session(sid) is None
+
+
+class TestDeleteSessionOrphansChildren:
+    def test_delete_orphans_children(self, db):
+        """Deleting a parent session orphans its children."""
+        db.create_session(session_id="parent", source="cli")
+        db.create_session(session_id="child", source="cli", parent_session_id="parent")
+        db.create_session(session_id="grandchild", source="cli", parent_session_id="child")
+
+        # Should not raise IntegrityError
+        result = db.delete_session("parent")
+        assert result is True
+        assert db.get_session("parent") is None
+        # Child is orphaned, not deleted
+        child = db.get_session("child")
+        assert child is not None
+        assert child["parent_session_id"] is None
+        # Grandchild is untouched
+        grandchild = db.get_session("grandchild")
+        assert grandchild is not None
+        assert grandchild["parent_session_id"] == "child"
+
 
 # =========================================================================
 # Schema and WAL mode