Migrate C# client-side reducer enum to the new syntax

As per clockworklabs/SpacetimeDBPrivate#1198.

This makes reducers use the same codegen structure as tables and other tuple types, but nests them inside a `Reducer` class as subclasses for pattern-matching, like we allow on tagged enums.

The reason we don't use a tagged enum here anymore is because we don't need BSATN sum type support for the Reducer enum, and it imposes unnecessary restrictions like max 256 elements.

.github/workflows/ci.yml

@@ -120,49 +120,26 @@ jobs:
         run: |
           sudo mkdir /stdb
           sudo chmod 777 /stdb
+
       - name: Checkout C# SDK
         uses: actions/checkout@v4
         with:
           repository: clockworklabs/spacetimedb-csharp-sdk
           ref: staging
           path: spacetimedb-csharp-sdk
 
-      - name: C# SDK tests
+      - name: Setup NuGet override for C# SDK
+        working-directory: spacetimedb-csharp-sdk
         run: |
-          ( cd crates/bindings-csharp/BSATN.Runtime && dotnet pack )
-          cd spacetimedb-csharp-sdk
-
-          # Write out the nuget config file to `nuget.config`. This causes the spacetimedb-csharp-sdk repository
-          # to be aware of the local versions of the `bindings-csharp` packages in SpacetimeDB, and use them if
-          # available. Otherwise, `spacetimedb-csharp-sdk` will use the NuGet versions of the packages.
-          # This means that (if version numbers match) we will test the local versions of the C# packages, even
-          # if they're not pushed to NuGet.
-          # See https://learn.microsoft.com/en-us/nuget/reference/nuget-config-file for more info on the config file,
-          # and https://tldp.org/LDP/abs/html/here-docs.html for more info on this bash feature.
-          cat >nuget.config <<EOF
-          <?xml version="1.0" encoding="utf-8"?>
-          <configuration>
-            <packageSources>
-              <!-- Local NuGet repositories -->
-              <add key="Local SpacetimeDB.BSATN.Runtime" value="../crates/bindings-csharp/BSATN.Runtime/bin/Release" />
-            </packageSources>
-            <packageSourceMapping>
-              <!-- Ensure that SpacetimeDB.BSATN.Runtime is used from the local folder. -->
-              <!-- Otherwise we risk an outdated version being quietly pulled from NuGet for testing. -->
-              <packageSource key="Local SpacetimeDB.BSATN.Runtime">
-                <package pattern="SpacetimeDB.BSATN.Runtime" />
-              </packageSource>
-              <!-- Fallback to NuGet for other packages. -->
-              <packageSource key="nuget.org">
-                <package pattern="*" />
-              </packageSource>
-            </packageSourceMapping>
-          </configuration>
-          EOF
+          dotnet pack ../crates/bindings-csharp/BSATN.Runtime
+          ./tools~/write-nuget-config.sh ..
 
           # clear package caches, so we get fresh ones even if version numbers haven't changed
           dotnet nuget locals all --clear
-          dotnet test
+
+      - name: Run C# SDK tests
+        working-directory: spacetimedb-csharp-sdk
+        run: dotnet test
 
   lints:
     name: Lints