Fix CS0168 warnings in generated code for unused explicitReader

Only declare the explicitReader variable in Decode methods when the type
actually has fields with explicit (non-implicit) context-specific tags.
Previously it was always declared, causing warnings for types like
KrbAlgorithmIdentifier, KrbSubjectPublicKeyInfo, GssapiToken, etc.
where all fields use implicit tags.

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

Author: SteveSyfuhs

Tests/Tests.Asn1SourceGenerator/Tests.Asn1SourceGenerator.csproj

@@ -5,7 +5,7 @@
     <LangVersion>12.0</LangVersion>
     <Nullable>enable</Nullable>
     <RootNamespace>Tests.Asn1SourceGenerator</RootNamespace>
-    <NoWarn>1701;1702</NoWarn>
+    <NoWarn>1591;1701;1702</NoWarn>
   </PropertyGroup>
 
   <ItemGroup>

Tools/Asn1SourceGenerator/Emit/CSharpEmitter.cs

@@ -497,7 +497,13 @@ private static void EmitDecodeGenericWithTag(IndentedWriter w, ResolvedType type
             w.WriteLine("decoded = new T();");
             w.WriteLine();
             w.WriteLine("AsnReader sequenceReader = reader.ReadSequence(expectedTag);");
-            w.WriteLine("AsnReader explicitReader;");
+
+            bool hasExplicitTags = type.Fields.Any(f => f.Encoding.TagNumber.HasValue && !f.Encoding.IsImplicit);
+
+            if (hasExplicitTags)
+            {
+                w.WriteLine("AsnReader explicitReader;");
+            }
 
             if (hasCollections)
             {