test_message_summary: Fix failing tests.

Tests were failing due to credit usage being capture in the next
month instead of current if the current time is last day of the
month. To fix this, we mock current time to not be the last day
of any month.

zerver/tests/test_message_summary.py

@@ -1,8 +1,10 @@
 import os
 import warnings
+from datetime import datetime, timezone
 from unittest import mock
 
 import orjson
+import time_machine
 from django.conf import settings
 from typing_extensions import override
 
@@ -44,12 +46,19 @@ class MessagesSummaryTestCase(ZulipTestCase):
             self.user, self.channel_name, content=content, topic_name=self.topic_name
         )
 
+        # Tests fail on the last day of the month due to us capturing the credit usage for that day
+        # on the first of the next month, so we need to set the date to a different day.
+        not_last_day_of_any_month = datetime(2025, 2, 18, 1, tzinfo=timezone.utc)
+
+        self.mocked_time_patcher = time_machine.travel(not_last_day_of_any_month, tick=False)
+        self.mocked_time_patcher.start()
         if settings.GENERATE_LITELLM_FIXTURES:  # nocoverage
             self.patcher = mock.patch("litellm.completion", wraps=litellm.completion)
             self.mocked_completion = self.patcher.start()
 
     @override
     def tearDown(self) -> None:
+        self.mocked_time_patcher.stop()
         if settings.GENERATE_LITELLM_FIXTURES:  # nocoverage
             self.patcher.stop()
         super().tearDown()