Reuse C# RPC process across CSharpRecipeTest methods (#7332)

Each test was shutting down and restarting the dotnet process, paying
the full cost of process startup + NuGet restore + JIT warmup every
time. On slow CI runners this can push individual tests past the
60-second RPC timeout, causing flaky failures.

Switch @AfterEach to resetCurrent() (clears RPC state, keeps process
alive) and add @afterall to shut down the process when the class is
done.

Author: steve-aom-elliott

rewrite-csharp/src/test/java/org/openrewrite/csharp/CSharpRecipeTest.java

@@ -15,6 +15,7 @@
  */
 package org.openrewrite.csharp;
 
+import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
@@ -71,6 +72,11 @@ static void setUpFactory() {
 
     @AfterEach
     void tearDown() {
+        CSharpRewriteRpc.resetCurrent();
+    }
+
+    @AfterAll
+    static void shutDown() {
         CSharpRewriteRpc.shutdownCurrent();
     }