test(channels): enforce member read-only posting permissions

tests/test_chat_message_flow.py

@@ -185,6 +185,36 @@ async def test_channel_member_cannot_delete_for_all(client, db_session):
     assert delete_for_all_by_member.status_code == 403
 
 
+async def test_channel_member_is_read_only_for_posting(client, db_session):
+    owner = await _create_verified_user(client, db_session, "channel_post_owner@example.com", "channel_post_owner", "strongpass123")
+    member = await _create_verified_user(client, db_session, "channel_post_member@example.com", "channel_post_member", "strongpass123")
+
+    me_member = await client.get("/api/v1/auth/me", headers={"Authorization": f"Bearer {member['access_token']}"})
+    member_id = me_member.json()["id"]
+
+    create_channel = await client.post(
+        "/api/v1/chats",
+        headers={"Authorization": f"Bearer {owner['access_token']}"},
+        json={"type": ChatType.CHANNEL.value, "title": "Read only channel", "member_ids": [member_id]},
+    )
+    assert create_channel.status_code == 200
+    chat_id = create_channel.json()["id"]
+
+    member_post = await client.post(
+        "/api/v1/messages",
+        headers={"Authorization": f"Bearer {member['access_token']}"},
+        json={"chat_id": chat_id, "type": "text", "text": "member post"},
+    )
+    assert member_post.status_code == 403
+
+    owner_post = await client.post(
+        "/api/v1/messages",
+        headers={"Authorization": f"Bearer {owner['access_token']}"},
+        json={"chat_id": chat_id, "type": "text", "text": "owner post"},
+    )
+    assert owner_post.status_code == 201
+
+
 async def test_group_invite_privacy_contacts_only(client, db_session):
     inviter = await _create_verified_user(client, db_session, "invite_u1@example.com", "invite_u1", "strongpass123")
     target = await _create_verified_user(client, db_session, "invite_u2@example.com", "invite_u2", "strongpass123")