fix: avoid OverflowException when alignment width is int.MinValue

Parsing a message template whose alignment is "-2147483648" (int.MinValue)
threw System.OverflowException from Math.Abs, violating the documented
contract that syntactically invalid templates yield text tokens rather
than throwing. Treat that alignment as text, matching how other alignment
values that cannot form a valid width are already handled.

test/Serilog.Tests/Parsing/MessageTemplateParserTests.cs

@@ -379,6 +379,13 @@ public class MessageTemplateParserTests
             new TextToken("{Hello,10-1}"));
     }
 
+    [Fact]
+    public void AlignmentOfIntMinValueIsParsedAsText()
+    {
+        AssertParsedAs("{Hello,-2147483648}",
+            new TextToken("{Hello,-2147483648}"));
+    }
+
     [Fact]
     public void EmptyAlignmentIsParsedAsText()
     {