fix(integrations): Correctly import MSTeams provider (#102973)

Was accidentally removed in
#99433, adds it back and tests
for it (and a small edit to import structure)

Fixes
[SENTRY-54V3](https://sentry.sentry.io/issues/6881966284/events/cf3fce57b76048ea88be2a7d57c7c502/)

Author: leeandher

src/sentry/integrations/msteams/__init__.py

@@ -1,5 +1,6 @@
 from sentry.integrations.msteams.spec import MsTeamsMessagingSpec
 
 from .handlers import MSTeamsActionHandler  # noqa: F401,F403
+from .notifications import *  # noqa: F401,F403
 
 MsTeamsMessagingSpec().initialize()

src/sentry/notifications/apps.py

@@ -5,14 +5,9 @@ class Config(AppConfig):
     name = "sentry.notifications"
 
     def ready(self) -> None:
-        import sentry.notifications.platform.sample  # NOQA
-
-        # Register the NotificationProviders for the platform
-        from sentry.notifications.platform.discord.provider import (  # NOQA
-            DiscordNotificationProvider,
-        )
-        from sentry.notifications.platform.email.provider import EmailNotificationProvider  # NOQA
-        from sentry.notifications.platform.msteams.provider import (  # NOQA
-            MSTeamsNotificationProvider,
-        )
-        from sentry.notifications.platform.slack.provider import SlackNotificationProvider  # NOQA
+        # Register the providers and templates for the new platform
+        import sentry.notifications.platform.discord.provider  # noqa: F401
+        import sentry.notifications.platform.email.provider  # noqa: F401
+        import sentry.notifications.platform.msteams.provider  # noqa: F401
+        import sentry.notifications.platform.slack.provider  # noqa: F401
+        import sentry.notifications.platform.templates  # noqa: F401

src/sentry/notifications/platform/templates/__init__.py

@@ -0,0 +1,4 @@
+# All templates should be imported here so they are registered in the notifications Django app.
+# See sentry/notifications/apps.py
+
+from .sample import *  # noqa: F401,F403

src/sentry/notifications/platform/sample.py renamed to src/sentry/notifications/platform/templates/sample.py

@@ -0,0 +1,31 @@
+from sentry.integrations.types import ExternalProviders
+from sentry.notifications.platform.types import NotificationProviderKey
+from sentry.testutils.cases import TestCase
+
+
+class NotificationsDjangoAppTest(TestCase):
+
+    def test_registers_legacy_providers(self) -> None:
+        """
+        This django app doesn't actually register these legacy providers because it would result
+        in some circular breakages from all the __init__.py imports. We'll still test it here
+        to make sure it doesn't break in the future.
+
+        If this test is failing for you, ensure all `@register_notification_provider` decorators
+        are triggered by an initialization import.
+        """
+        from sentry.notifications.notify import registry
+
+        assert len(registry) == 3
+        assert registry[ExternalProviders.EMAIL] is not None
+        assert registry[ExternalProviders.SLACK] is not None
+        assert registry[ExternalProviders.MSTEAMS] is not None
+
+    def test_registers_platform_providers(self) -> None:
+        from sentry.notifications.platform.registry import provider_registry
+
+        assert len(provider_registry.registrations) == 4
+        assert provider_registry.get(NotificationProviderKey.DISCORD) is not None
+        assert provider_registry.get(NotificationProviderKey.EMAIL) is not None
+        assert provider_registry.get(NotificationProviderKey.MSTEAMS) is not None
+        assert provider_registry.get(NotificationProviderKey.SLACK) is not None