Mono: Fix nullable annotation CI failure in ScriptPathAttribute

Remove nullable annotation from SourceFile property since the project
does not enable nullable context. Use plain string with set accessor.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: matelq

modules/mono/csharp_script.cpp

@@ -2847,7 +2847,7 @@ Ref<Resource> ResourceFormatLoaderCSharpScript::load(const String &p_path, const
 	// TODO ignore anything inside bin/ and obj/ in tools builds?
 
 	String real_path = p_path;
-	bool is_assembly_backed = false;
+	[[maybe_unused]] bool is_assembly_backed = false;
 	if (p_path.begins_with("csharp://")) {
 		String virtual_suffix = p_path.trim_prefix("csharp://");
 		int colon_idx = virtual_suffix.find(":");

modules/mono/glue/GodotSharp/GodotSharp/Core/Attributes/ScriptPathAttribute.cs

@@ -14,11 +14,10 @@ public sealed class ScriptPathAttribute : Attribute
         public string Path { get; }
 
         /// <summary>
-        /// Absolute path to the original source file on disk. Only set for assembly-backed
-        /// scripts (csharp:// paths) where the source file exists outside the Godot project.
-        /// Null for res:// scripts and NuGet packages (where source is not available).
+        /// Absolute path to the original source file on disk.
+        /// Null for res:// scripts and NuGet packages.
         /// </summary>
-        public string? SourceFile { get; init; }
+        public string SourceFile { get; set; }
 
         /// <summary>
         /// Constructs a new ScriptPathAttribute instance.