Address Copilot review feedback

- Remove the redundant second `TrimEnd('\n')` from the return path.
  The earlier trim already removes any trailing newline before the
  `\n...` doc-end check; the marker-removal substring does not
  re-introduce one, so the second trim was dead code.
- Surface full exception (`ex.ToString()`) in the test round-trip
  helper so YAML parse failures show stack + inner exception, not
  just the top-level message.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/Runner.Worker/Dap/YamlScalarFormatter.cs

@@ -57,7 +57,7 @@ namespace GitHub.Runner.Worker.Dap
             {
                 raw = raw.Substring(0, raw.Length - DocEndMarker.Length);
             }
-            return raw.TrimEnd('\n');
+            return raw;
         }
     }
 }

src/Test/L0/Worker/YamlScalarFormatterL0.cs

@@ -27,7 +27,7 @@ namespace GitHub.Runner.Common.Tests.Worker
             catch (Exception ex)
             {
                 throw new Xunit.Sdk.XunitException(
-                    $"Formatted scalar did not round-trip as valid YAML.\nInput: '{value}'\nFormatted: '{scalar}'\nFull YAML:\n{yaml}\nError: {ex.Message}");
+                    $"Formatted scalar did not round-trip as valid YAML.\nInput: '{value}'\nFormatted: '{scalar}'\nFull YAML:\n{yaml}\nError: {ex}");
             }
             Assert.NotNull(doc);
             Assert.True(doc.ContainsKey("k"), $"missing key in parsed doc. Formatted: '{scalar}'");