fix(gateway): make WriteEngine + converter raise-safe against malformed write commands

tests/unit/engines/test_write.py

@@ -161,6 +161,46 @@ async def test_write_bad_type_returns_error_without_raising():
     )
 
 
+@pytest.mark.asyncio
+async def test_write_none_data_type_returns_error_without_raising():
+    eng = make_engine()
+
+    result = await eng.write(base_cmd(data_type=None))
+
+    assert result.ok is False
+    assert result.status == 'error'
+    eng.sink.update_write_status.assert_awaited_once()
+    eng.pool.get.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_write_missing_value_key_returns_error_without_raising():
+    eng = make_engine()
+    cmd = base_cmd()
+    del cmd['value']
+
+    result = await eng.write(cmd)
+
+    assert result.ok is False
+    assert result.status == 'error'
+    eng.sink.update_write_status.assert_awaited_once()
+    eng.pool.get.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_write_missing_data_type_key_returns_error_without_raising():
+    eng = make_engine()
+    cmd = base_cmd()
+    del cmd['data_type']
+
+    result = await eng.write(cmd)
+
+    assert result.ok is False
+    assert result.status == 'error'
+    eng.sink.update_write_status.assert_awaited_once()
+    eng.pool.get.assert_not_awaited()
+
+
 @pytest.mark.asyncio
 async def test_write_value_raises_maps_to_error_result_not_raised():
     eng = make_engine()