fix: monthly tasks scheduled for day 29-31 skip every short month (#1668)

tests/test_compute_next_run_monthly_clamp.py

@@ -0,0 +1,56 @@
+"""compute_next_run monthly must clamp to short months, not skip them.
+
+Old behavior: now.replace(day=31) raises ValueError in February, the
+except set candidate = now, candidate <= now then jumped straight to the
+NEXT month (which does clamp). A task scheduled for day 31 therefore never
+fired in February, April, June, September or November.
+"""
+
+from datetime import datetime
+
+import pytest
+
+from src.task_scheduler import compute_next_run
+
+
+@pytest.mark.parametrize(
+    "day,after,expected",
+    [
+        (31, datetime(2026, 2, 15, 12, 0), datetime(2026, 2, 28, 9, 0)),
+        (30, datetime(2026, 2, 1, 12, 0), datetime(2026, 2, 28, 9, 0)),
+        (29, datetime(2026, 2, 1, 12, 0), datetime(2026, 2, 28, 9, 0)),
+        (29, datetime(2028, 2, 1, 12, 0), datetime(2028, 2, 29, 9, 0)),
+        (31, datetime(2026, 4, 1, 12, 0), datetime(2026, 4, 30, 9, 0)),
+    ],
+)
+def test_monthly_clamps_to_last_day_of_current_short_month(day, after, expected):
+    out = compute_next_run("monthly", "09:00", scheduled_day=day, after=after)
+    assert out == expected
+
+
+def test_monthly_clamped_slot_already_passed_rolls_to_next_month():
+    out = compute_next_run(
+        "monthly", "09:00", scheduled_day=31, after=datetime(2026, 2, 28, 10, 0)
+    )
+    assert out == datetime(2026, 3, 31, 9, 0)
+
+
+def test_monthly_regular_day_still_fires_this_month():
+    out = compute_next_run(
+        "monthly", "09:00", scheduled_day=15, after=datetime(2026, 6, 10, 12, 0)
+    )
+    assert out == datetime(2026, 6, 15, 9, 0)
+
+
+def test_monthly_regular_day_passed_rolls_to_next_month():
+    out = compute_next_run(
+        "monthly", "09:00", scheduled_day=15, after=datetime(2026, 6, 20, 12, 0)
+    )
+    assert out == datetime(2026, 7, 15, 9, 0)
+
+
+def test_monthly_december_year_rollover():
+    out = compute_next_run(
+        "monthly", "09:00", scheduled_day=31, after=datetime(2026, 12, 31, 10, 0)
+    )
+    assert out == datetime(2027, 1, 31, 9, 0)