DYN-9702: Log when shared definitions folder is missing

Add a Trace.TraceWarning in PathManager.DefinitionDirectories when the
common "definitions" folder (containing Curve_Validate.dyf, copied by
DynamoCore.csproj at build time) is not found at commonDataDir. This
is diagnostic-only -- the returned directory list is unchanged, and we
deliberately do not auto-create the folder (that approach was tried
and reverted). Previously this case failed completely silently, which
caused a hard-to-diagnose CI regression. PathManager has no injected
logger, so this follows the existing Trace-based diagnostic convention
used elsewhere in the codebase for logger-less, non-fatal issues
rather than adding a new logging dependency.

Author: jasonstratton

src/DynamoCore/Configuration/PathManager.cs

@@ -225,6 +225,12 @@ public IEnumerable<string> DefinitionDirectories
                 {
                     definitionDirectories.Add(commonDefinitionsDirectory);
                 }
+                else if (!Directory.Exists(commonDefinitionsDirectory))
+                {
+                    // Diagnostic only: log the missing shared definitions folder so the omission
+                    // is visible. Does not change the returned directories.
+                    Trace.TraceWarning("Expected shared definitions folder not found at: " + commonDefinitionsDirectory);
+                }
 
                 return definitionDirectories;
             }