Fix delegate return type: use byte* instead of string for const char*

Fixes #271 - Delegates (function pointers in structs like AVClass.item_name)
incorrectly used string return type for const char*. String marshaling does
not work with unmanaged function pointers, causing AccessViolationException.

Changed generator to emit byte* for delegate return types when the C
function returns const char*.

Author: Ruslan-B

FFmpeg.AutoGen.Abstractions/generated/Delegates.g.cs

@@ -106,7 +106,7 @@ public unsafe struct AVClass_get_category_func
 }
 
 [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
-public unsafe delegate string AVClass_item_name (void* @ctx);
+public unsafe delegate byte* AVClass_item_name (void* @ctx);
 public unsafe struct AVClass_item_name_func
 {
     public IntPtr Pointer;

FFmpeg.AutoGen.CppSharpUnsafeGenerator/Processing/FunctionProcessor.cs

@@ -82,14 +82,29 @@ internal TypeDefinition GetDelegateType(FunctionType functionType, string name)
         {
             Name = $"{name}_func",
             FunctionName = name,
-            ReturnType = GetReturnType(functionType.ReturnType.Type, name),
+            ReturnType = GetDelegateReturnType(functionType.ReturnType.Type, name),
             Parameters = functionType.Parameters.Select(GetParameter).ToArray()
         };
         _context.AddDefinition(@delegate);
 
         return @delegate;
     }
 
+    private TypeDefinition GetDelegateReturnType(Type type, string name)
+    {
+        // For delegates (function pointers), const char* must be byte* not string,
+        // because string marshaling does not work with unmanaged function pointers.
+        if (type is PointerType pointerType &&
+            pointerType.QualifiedPointee.Qualifiers.IsConst &&
+            pointerType.Pointee is BuiltinType builtinType &&
+            builtinType.Type == PrimitiveType.Char)
+        {
+            return new TypeDefinition { Name = "byte*" };
+        }
+
+        return GetReturnType(type, name);
+    }
+
     private FunctionParameter GetParameter(Parameter parameter, int position)
     {
         var name = string.IsNullOrEmpty(parameter.Name) ? $"p{position}" : parameter.Name;

FFmpeg.AutoGen/generated/Delegates.g.cs

@@ -106,7 +106,7 @@ public unsafe struct AVClass_get_category_func
 }
 
 [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
-public unsafe delegate string AVClass_item_name (void* @ctx);
+public unsafe delegate byte* AVClass_item_name (void* @ctx);
 public unsafe struct AVClass_item_name_func
 {
     public IntPtr Pointer;