Fix reading relative path file reference

A reference like ./Directory/File.json#/components/schema/abc was before read as:
refId = abc
externalResource = ./Directory

Which dropped the file name, which will be fixed by this commit

Author: dldl-cmd

src/Microsoft.OpenApi/Reader/V3/OpenApiV3Deserializer.cs

@@ -193,7 +193,11 @@ private static (string, string) GetReferenceIdAndExternalResource(string pointer
             var refId = refSegments.Last();
             var isExternalResource = !refSegments.First().StartsWith("#", StringComparison.OrdinalIgnoreCase);
 
-            string externalResource = isExternalResource ? $"{refSegments.First()}/{refSegments[1].TrimEnd('#')}" : null;
+            string externalResource = null;
+            if (isExternalResource)
+            {
+                externalResource = pointer.Split('#').FirstOrDefault()?.TrimEnd('#');
+            }
 
             return (refId, externalResource);
         }

src/Microsoft.OpenApi/Reader/V31/OpenApiV31Deserializer.cs

@@ -165,7 +165,7 @@ private static (string, string) GetReferenceIdAndExternalResource(string pointer
             string externalResource = null;
             if (isExternalResource && pointer.Contains('#'))
             {
-                externalResource = $"{refSegments.First()}/{refSegments[1].TrimEnd('#')}";
+                externalResource = pointer.Split('#').FirstOrDefault()?.TrimEnd('#');
             }
 
             return (refId, externalResource);