[C#] Mark experimental API (#1769)

crates/bindings-csharp/Codegen.Tests/fixtures/server/Lib.cs

@@ -2,6 +2,7 @@
 using SpacetimeDB;
 
 #pragma warning disable CA1050 // Declare types in namespaces - this is a test fixture, no need for a namespace.
+#pragma warning disable STDB_UNSTABLE // Enable experimental SpacetimeDB features
 
 [SpacetimeDB.Type]
 public partial struct CustomStruct

crates/bindings-csharp/Codegen/Module.cs

@@ -451,8 +451,13 @@ public Scope.Extensions GenerateSchedule()
     {
         var extensions = new Scope.Extensions(Scope, FullName);
 
+        // Mark the API as unstable. We use name `STDB_UNSTABLE` because:
+        // 1. It's a close equivalent of the `unstable` Cargo feature in Rust.
+        // 2. Our diagnostic IDs use either BSATN or STDB prefix depending on the package.
+        // 3. We don't expect to mark individual experimental features with numeric IDs, so we don't use the standard 1234 suffix.
         extensions.Contents.Append(
             $$"""
+            [System.Diagnostics.CodeAnalysis.Experimental("STDB_UNSTABLE")]
             public static void VolatileNonatomicScheduleImmediate{{Name}}({{string.Join(
                 ", ",
                 Args.Select(a => $"{a.Type} {a.Name}")