migration: Set empty subject for DMs and GDMs from Slack.

This migration updates the `subject´ field to an empty string for
DMs and GDMs imported from Slack. This change ensures the previously
converted messages aligns with Zulip standards where private
messages don't have subjects.

zerver/migrations/0625_empty_subject_for_slack_dms.py

@@ -0,0 +1,38 @@
+# Generated by Django 5.0.9 on 2024-11-08 13:02
+
+from django.db import migrations
+from django.db.backends.base.schema import BaseDatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+
+def update_dm_and_gdm_topic_name(apps: StateApps, schema_editor: BaseDatabaseSchemaEditor) -> None:
+    RECIPIENT_PERSONAL = 1
+    RECIPIENT_HUDDLE = 3
+    CLIENT_ID = 1
+    Message = apps.get_model("zerver", "Message")
+
+    # Matches subjects like '2015-08-18 Slack thread 2'
+    slack_thread_pattern = r"\d{4}-\d{2}-\d{2} Slack thread \d+"
+
+    messages_to_update = Message.objects.filter(
+        sending_client_id=CLIENT_ID,
+        recipient__type__in=[RECIPIENT_PERSONAL, RECIPIENT_HUDDLE],
+        subject="Imported from Slack",
+    ) | Message.objects.filter(
+        sending_client_id=CLIENT_ID,
+        recipient__type__in=[RECIPIENT_PERSONAL, RECIPIENT_HUDDLE],
+        subject__regex=slack_thread_pattern,
+    )
+
+    messages_to_update.update(subject="")
+
+
+class Migration(migrations.Migration):
+    atomic = False
+    dependencies = [
+        ("zerver", "0624_alter_realmexport_tarball_size_bytes"),
+    ]
+
+    operations = [
+        migrations.RunPython(update_dm_and_gdm_topic_name),
+    ]