Fix FormattedMessage not escaping text when converted to markup

Meant that ToMarkup() wasn't actually round-trip safe.

RELEASE-NOTES.md

@@ -44,7 +44,7 @@ END TEMPLATE-->
 
 ### Bugfixes
 
-*None yet*
+* Fixed `FormattedMessage` not escaping plain text content properly with `.ToMarkup()`.
 
 ### Other
 

Robust.Shared/Utility/MarkupNode.cs

@@ -35,7 +35,7 @@ public sealed class MarkupNode : IComparable<MarkupNode>, IEquatable<MarkupNode>
     public override string ToString()
     {
         if(Name == null)
-            return Value.StringValue ?? "";
+            return FormattedMessage.EscapeText(Value.StringValue ?? "");
 
         var attributesString = "";
         foreach (var (k, v) in Attributes)

Robust.UnitTesting/Shared/Utility/FormattedMessage_Test.cs

@@ -83,5 +83,18 @@ namespace Robust.UnitTesting.Shared.Utility
                 message.EnumerateRunes(),
                 Is.EquivalentTo(message.ToString().EnumerateRunes()));
         }
+
+        /// <summary>
+        /// Test that the given formatted message string provides equal result when output & parsed again.
+        /// </summary>
+        [Test]
+        [TestCase("\\[whaaaaa")]
+        public static void TestRoundTrip(string markup)
+        {
+            var message = FormattedMessage.FromMarkupOrThrow(markup);
+            var secondMessage = FormattedMessage.FromMarkupOrThrow(message.ToMarkup());
+
+            Assert.That(secondMessage, NUnit.Framework.Is.EqualTo(message));
+        }
     }
 }