test(signal_v2): add scheduler boundary tests at exact transitions

Code review noted missing boundary tests at:
- 09:00:00 (pre-market → market) → 60
- 15:30:00 (market → post-market) → 300
- 20:00:00 (post-market → overnight skip)

3 new tests, total 8 scheduler / 14 signal_v2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

signal_v2/tests/test_scheduler.py

@@ -39,3 +39,23 @@ def test_next_interval_holiday_skip():
     interval = _next_interval(now)
     # Next: 2026-05-06 (Wed) 07:00, ~21h away
     assert 20 * 3600 < interval < 22 * 3600
+
+
+def test_next_interval_at_market_open_boundary():
+    """09:00:00 정확 second → 60초 (market 구간 진입)."""
+    now = _kst(2026, 5, 18, 9, 0)  # Monday 09:00:00
+    assert _next_interval(now) == 60
+
+
+def test_next_interval_at_market_close_boundary():
+    """15:30:00 정확 second → 300초 (post-market 구간 진입)."""
+    now = _kst(2026, 5, 18, 15, 30)  # Monday 15:30:00
+    assert _next_interval(now) == 300
+
+
+def test_next_interval_at_polling_window_end_boundary():
+    """20:00:00 정확 second → overnight skip (다음 영업일 07:00 까지)."""
+    now = _kst(2026, 5, 18, 20, 0)  # Monday 20:00:00
+    interval = _next_interval(now)
+    # Next: Tuesday 07:00 — 11h away
+    assert 11 * 3600 - 60 < interval < 11 * 3600 + 60