Fix "Last day of the month" schedules in dagster (#25169)

## Summary & Motivation
The is_numeric check here was incorrectly assumimg that if it wasn't
"*", it would be a number (and also assuming that it would still be a
string even if it was numeric, which was more harmless). But you can
have "L" in a cronstring to indicate the last day of the month.

## How I Tested These Changes
BK, load UI with one of these schedules and turn it on

## Changelog

- [x] `BUGFIX` Fixed an issue where schedules with "L" in the cronstring
(to indicate the last day of the month) would raise an error in the
Dagster UI.

python_modules/dagster/dagster/_utils/schedules.py

@@ -637,7 +637,7 @@ def cron_string_iterator(
     # Croniter >= 1.4 returns 3 items
     cron_parts, nth_weekday_of_month, *_ = CroniterShim.expand(cron_string)
 
-    is_numeric = [len(part) == 1 and part[0] != "*" for part in cron_parts]
+    is_numeric = [len(part) == 1 and isinstance(part[0], int) for part in cron_parts]
     is_wildcard = [len(part) == 1 and part[0] == "*" for part in cron_parts]
 
     all_numeric_minutes = len(cron_parts[0]) > 0 and all(
@@ -657,7 +657,7 @@ def cron_string_iterator(
         if (
             all(is_numeric[0:3])
             and all(is_wildcard[3:])
-            and int(cron_parts[2][0]) <= MAX_DAY_OF_MONTH_WITH_GUARANTEED_MONTHLY_INTERVAL
+            and cron_parts[2][0] <= MAX_DAY_OF_MONTH_WITH_GUARANTEED_MONTHLY_INTERVAL
         ):  # monthly
             known_schedule_type = ScheduleType.MONTHLY
         elif all(is_numeric[0:2]) and is_numeric[4] and all(is_wildcard[2:4]):  # weekly
@@ -668,16 +668,16 @@ def cron_string_iterator(
             known_schedule_type = ScheduleType.HOURLY
 
     if is_numeric[1]:
-        expected_hour = int(cron_parts[1][0])
+        expected_hour = cron_parts[1][0]
 
     if all_numeric_minutes:
-        expected_minutes = [int(cron_part) for cron_part in cron_parts[0]]
+        expected_minutes = [cron_part for cron_part in cron_parts[0]]
 
     if is_numeric[2]:
-        expected_day = int(cron_parts[2][0])
+        expected_day = cron_parts[2][0]
 
     if is_numeric[4]:
-        expected_day_of_week = int(cron_parts[4][0])
+        expected_day_of_week = cron_parts[4][0]
 
     if known_schedule_type:
         start_datetime = datetime.datetime.fromtimestamp(

python_modules/dagster/dagster_tests/scheduler_tests/test_cron_string_iterator.py

@@ -582,3 +582,38 @@ def test_reversed_dst_transition_advances(execution_timezone, cron_string, times
                 prev_time = next_time
 
             start_timestamp = start_timestamp - timestamp_interval
+
+
+def test_last_day_of_month_cron_schedule():
+    # L means last day of month
+    execution_timezone = "Europe/Berlin"
+    cron_string = "*/15 13 L * *"
+
+    expected_datetimes = [
+        create_datetime(2023, 10, 31, 13, 0, 0, tz="Europe/Berlin"),
+        create_datetime(2023, 10, 31, 13, 15, 0, tz="Europe/Berlin"),
+        create_datetime(2023, 10, 31, 13, 30, 0, tz="Europe/Berlin"),
+        create_datetime(2023, 10, 31, 13, 45, 0, tz="Europe/Berlin"),
+        create_datetime(2023, 11, 30, 13, 0, 0, tz="Europe/Berlin"),
+        create_datetime(2023, 11, 30, 13, 15, 0, tz="Europe/Berlin"),
+        create_datetime(2023, 11, 30, 13, 30, 0, tz="Europe/Berlin"),
+        create_datetime(2023, 11, 30, 13, 45, 0, tz="Europe/Berlin"),
+        create_datetime(2023, 12, 31, 13, 0, 0, tz="Europe/Berlin"),
+        create_datetime(2023, 12, 31, 13, 15, 0, tz="Europe/Berlin"),
+        create_datetime(2023, 12, 31, 13, 30, 0, tz="Europe/Berlin"),
+        create_datetime(2023, 12, 31, 13, 45, 0, tz="Europe/Berlin"),
+    ]
+
+    start_timestamp = expected_datetimes[0].timestamp() - 1
+
+    cron_iter = cron_string_iterator(start_timestamp, cron_string, execution_timezone)
+
+    for i in range(len(expected_datetimes)):
+        assert next(cron_iter) == expected_datetimes[i]
+
+    end_timestamp = expected_datetimes[-1].timestamp() + 1
+
+    cron_iter = reverse_cron_string_iterator(end_timestamp, cron_string, execution_timezone)
+
+    for i in range(len(expected_datetimes)):
+        assert next(cron_iter) == expected_datetimes[-(i + 1)]