Add durable platform event delivery contract

tests/test_install_config.py

@@ -21,6 +21,30 @@ class InstallConfigTests(unittest.TestCase):
         with self.assertRaises(ValidationError):
             Settings(CALENDAR_OUTBOX_TERMINAL_RETENTION_DAYS="-1")
 
+    def test_platform_event_outbox_limits_are_validated(self) -> None:
+        defaults = Settings()
+        self.assertEqual(defaults.platform_event_outbox_max_attempts, 8)
+        self.assertEqual(
+            defaults.platform_event_outbox_terminal_retention_days,
+            90,
+        )
+        configured = Settings(
+            PLATFORM_EVENT_OUTBOX_MAX_ATTEMPTS="12",
+            PLATFORM_EVENT_OUTBOX_TERMINAL_RETENTION_DAYS="30",
+        )
+        self.assertEqual(configured.platform_event_outbox_max_attempts, 12)
+        self.assertEqual(
+            configured.platform_event_outbox_terminal_retention_days,
+            30,
+        )
+        for values in (
+            {"PLATFORM_EVENT_OUTBOX_MAX_ATTEMPTS": "0"},
+            {"PLATFORM_EVENT_OUTBOX_MAX_ATTEMPTS": "101"},
+            {"PLATFORM_EVENT_OUTBOX_TERMINAL_RETENTION_DAYS": "-1"},
+        ):
+            with self.subTest(values=values), self.assertRaises(ValidationError):
+                Settings(**values)
+
     def test_scheduling_cancellation_notice_setting_is_bounded(self) -> None:
         self.assertEqual(Settings().scheduling_cancellation_notice_days, 30)
         self.assertEqual(