fix(authoring): CSV import accepts variable-length arrays; Plain MQTT isArray warns at authoring (lmxopcua#770)

CSV: IsArray with blank/0 ArrayLength is the variable-length shape browse-commit and the composer produce, so its own export now re-imports.
Plain MQTT: MqttTagDefinitionFactory.Inspect (the driver's parse, consulted by the authoring inspector) now advises when isArray is set on a Plain-shaped tag; Sparkplug-shaped blobs stay quiet.
Live Sparkplug array test and SDK reflection items remain open.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Tests/MqttTagDefinitionFactoryTests.cs

@@ -72,6 +72,18 @@ public sealed class MqttTagDefinitionFactoryTests
         => MqttTagDefinitionFactory.FromTagConfig(
             """{"topic":"a/b","payloadFormat":"Json","dataType":"Double"}""", RawPath, out _).ShouldBeFalse();
 
+    [Fact]
+    public void Inspect_IsArrayOnPlainTag_Warns_lmxopcua_770()
+    {
+        var warnings = MqttTagDefinitionFactory.Inspect("""{"topic":"a/b","payloadFormat":"Json","isArray":true}""");
+        warnings.ShouldHaveSingleItem().ShouldContain("isArray");
+    }
+
+    [Fact]
+    public void Inspect_IsArrayOnSparkplugTag_StaysQuiet()
+        => MqttTagDefinitionFactory.Inspect(
+            """{"groupId":"G","edgeNodeId":"E","metricName":"M","topic":"leftover","isArray":true}""").ShouldBeEmpty();
+
     [Fact]
     public void Inspect_WildcardTopic_ReturnsWarning()
     {