fix(generator): use tempDict for ReadOnlyDictionary with unmanaged KVP

When deserializing ReadOnlyDictionary with unmanaged key-value pairs,
the generated code was incorrectly trying to assign to value[kvp.Key]
which fails because ReadOnlyDictionary doesn't support indexer
assignment. Now correctly assigns to tempDict instead.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: JasonXuDeveloper

src/Nino.Generator/BuiltInType/DictionaryGenerator.cs

@@ -191,7 +191,15 @@ protected override void GenerateDeserializer(ITypeSymbol typeSymbol, Writer writ
             writer.Append("        reader.UnsafeRead(out ");
             writer.Append(kvpTypeName);
             writer.AppendLine(" kvp);");
-            writer.AppendLine("        value[kvp.Key] = kvp.Value;");
+
+            if (isReadOnlyDictionary)
+            {
+                writer.AppendLine("        tempDict[kvp.Key] = kvp.Value;");
+            }
+            else
+            {
+                writer.AppendLine("        value[kvp.Key] = kvp.Value;");
+            }
         }
         else
         {