Update fields to be readonly

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/V2/OpenApiV2VersionService.cs

@@ -31,7 +31,7 @@ public OpenApiV2VersionService(OpenApiDiagnostic diagnostic)
             Diagnostic = diagnostic;
         }
 
-        private Dictionary<Type, Func<ParseNode, OpenApiDocument, object>> _loaders = new()
+        private readonly Dictionary<Type, Func<ParseNode, OpenApiDocument, object>> _loaders = new()
         {
             [typeof(OpenApiAny)] = OpenApiV2Deserializer.LoadAny,
             [typeof(OpenApiContact)] = OpenApiV2Deserializer.LoadContact,

src/Microsoft.OpenApi.Readers/V3/OpenApiV3VersionService.cs

@@ -34,7 +34,7 @@ public OpenApiV3VersionService(OpenApiDiagnostic diagnostic)
             Diagnostic = diagnostic;
         }
 
-        private Dictionary<Type, Func<ParseNode, OpenApiDocument, object>> _loaders = new()
+        private readonly Dictionary<Type, Func<ParseNode, OpenApiDocument, object>> _loaders = new()
         {
             [typeof(OpenApiAny)] = OpenApiV3Deserializer.LoadAny,
             [typeof(OpenApiCallback)] = OpenApiV3Deserializer.LoadCallback,

src/Microsoft.OpenApi/Services/HostDocumentResolver.cs

@@ -8,7 +8,7 @@ namespace Microsoft.OpenApi.Services
 {
     internal class HostDocumentResolver : OpenApiVisitorBase
     {
-        private OpenApiDocument _currentDocument;
+        private readonly OpenApiDocument _currentDocument;
 
         public HostDocumentResolver(OpenApiDocument currentDocument)
         {