add reflection-based test coverage for DiscoveryService.GetModelNameFromId

Ensures the 0x20 => "POS895" registration is actually tested. Without this,
the line could be deleted and the whole suite would still pass.

Claude-Session: https://claude.ai/code/session_01E6N3P4nSZLtp4BC2LXVref

Inspectron.Epson.Tests/PrinterRegistrationTests.cs

@@ -1,3 +1,4 @@
+using Inspectron.Epson;
 using Inspectron.Epson.PrintServer.Printers;
 using Inspectron.Epson.PrintServer.Printers.Utils;
 
@@ -82,4 +83,40 @@ public class PrinterRegistrationTests
             Assert.NotNull(factory.Create((int)type, 0x20));
         }
     }
+
+    [Fact]
+    public void GetModelNameFromId_maps_0x20_to_POS895()
+    {
+        Assert.Equal("POS895", ModelNameFor(0x20));
+    }
+
+    [Fact]
+    public void GetModelNameFromId_still_maps_the_existing_epson_ids()
+    {
+        // Guards against a change that makes every id report as POS895.
+        Assert.Equal("TM-T30III", ModelNameFor(0x01));
+        Assert.Equal("TM-U220II", ModelNameFor(0x0D));
+        Assert.Equal("TM-U220II", ModelNameFor(0x13));
+    }
+
+    [Fact]
+    public void GetModelNameFromId_falls_back_for_an_unknown_id()
+    {
+        Assert.Equal("Unknown (0x99)", ModelNameFor(0x99));
+    }
+
+    /// <summary>
+    /// Invokes DiscoveryService.GetModelNameFromId, which is private static and
+    /// otherwise reachable only through a live TCP connection. Reflection keeps the
+    /// production code untouched; if the method is renamed this fails loudly rather
+    /// than silently passing.
+    /// </summary>
+    private static string ModelNameFor(byte printerId)
+    {
+        var method = typeof(DiscoveryService).GetMethod("GetModelNameFromId",
+            System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic);
+
+        Assert.NotNull(method);
+        return (string)method!.Invoke(null, new object[] { printerId })!;
+    }
 }