#131 Add the missing InvalidHostnames key to Core's Resources.resx (#133)

The strongly-typed Resources.InvalidHostnames getter existed but Core's
Resources.resx had no matching <data name="InvalidHostnames"> entry, so it
resolved to null. HostsEntry.ValidateHostnames does
`SetError(nameof(HostNames), !_hostnamesValid ? Resources.InvalidHostnames : null)`
and SetError treats null as "clear the error", so an invalid hostname surfaced a
BLANK IDataErrorInfo message in both the classic and modern editions instead of
"Invalid host names".

Add the key ("Invalid host names", matching the WinForm resource) plus a
regression test asserting the invalid-hostname error message is the non-empty
resource string. Split out of the larger CI/coverage PR #132 so the user-facing
fix can land on its own.

Closes #131

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: scottlerch

HostsFileEditor.Core.Tests/HostsEntryTests.cs

@@ -81,6 +81,20 @@ public void Invalid_Hostname_AfterEdit()
         entry.Valid.ShouldBeFalse();
     }
 
+    // Regression for #131: Resources.InvalidHostnames had no matching key in Core's Resources.resx,
+    // so it resolved to null and SetError(null) cleared the error — an invalid hostname surfaced a
+    // BLANK IDataErrorInfo message (rather than "Invalid host names") in both editions.
+    [TestMethod]
+    public void Invalid_Hostname_HasNonBlankErrorMessage()
+    {
+        var entry = new HostsEntry("127.0.0.1 goodhost");
+        entry.HostNames = "bad@host"; // invalid character
+
+        entry.Valid.ShouldBeFalse();
+        entry["HostNames"].ShouldNotBeNullOrEmpty();
+        entry["HostNames"].ShouldBe(HostsFileEditor.Properties.Resources.InvalidHostnames);
+    }
+
     [TestMethod]
     public void ToString_ReturnsExpected()
     {