fix(adminui): S7 typed page no longer wipes Tags on save

- S7DriverPage.FormModel now preserves Tags through Form ↔ Options
  translation (was hard-coding Tags = [] on every save, silently
  destroying any tag list that operators had configured).
- Add FormModel_RoundTrip tests for OpcUaClient and Historian
  mirror classes — both were translating Options ↔ form-model
  entirely untested.
- Surface S7 Tags in the round-trip test so this regression
  can't reach merge again.

tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/S7DriverPageFormSerializationTests.cs

@@ -69,6 +69,12 @@ public sealed class S7DriverPageFormSerializationTests
     [Fact]
     public void FormModel_RoundTrip_PreservesEditableFields()
     {
+        var tags = new[]
+        {
+            new S7TagDefinition("Speed",  "DB1.DBD0",  S7DataType.Float32, Writable: true),
+            new S7TagDefinition("Status", "DB1.DBW4",  S7DataType.Int16,   Writable: false),
+        };
+
         var opts = new S7DriverOptions
         {
             Host = "192.168.1.50",
@@ -84,6 +90,7 @@ public sealed class S7DriverPageFormSerializationTests
                 Timeout = TimeSpan.FromSeconds(4),
             },
             ProbeTimeoutSeconds = 20,
+            Tags = tags,
         };
 
         var form = ZB.MOM.WW.OtOpcUa.AdminUI.Components.Pages.Clusters.Drivers
@@ -100,5 +107,14 @@ public sealed class S7DriverPageFormSerializationTests
         roundTripped.Probe.Interval.ShouldBe(TimeSpan.FromSeconds(8));
         roundTripped.Probe.Timeout.ShouldBe(TimeSpan.FromSeconds(4));
         roundTripped.ProbeTimeoutSeconds.ShouldBe(20);
+
+        // Tags must survive the FormModel round-trip unchanged (regression guard for the
+        // Tags = [] data-loss bug fixed in this PR).
+        roundTripped.Tags.Count.ShouldBe(2);
+        roundTripped.Tags[0].Name.ShouldBe("Speed");
+        roundTripped.Tags[0].Address.ShouldBe("DB1.DBD0");
+        roundTripped.Tags[0].DataType.ShouldBe(S7DataType.Float32);
+        roundTripped.Tags[1].Name.ShouldBe("Status");
+        roundTripped.Tags[1].Writable.ShouldBeFalse();
     }
 }