refactor: Convert DefaultExecutorProvider to enum singleton

Changed from utility class pattern to enum singleton pattern. This ensures
the executor is created once and reused across all CopilotClient instances,
which is especially beneficial for the virtual thread executor on JDK 25+.

Benefits:
- Single virtual thread executor shared across all clients (more efficient)
- Thread-safe lazy initialization (enum guarantee)
- Prevents instantiation without needing private constructor

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: brunoborges

.vscode/mcp.json

@@ -157,7 +157,7 @@ public CopilotClient(CopilotClientOptions options) {
         }
 
         Executor providedExecutor = this.options.getExecutor();
-        this.executor = providedExecutor != null ? providedExecutor : DefaultExecutorProvider.create();
+        this.executor = providedExecutor != null ? providedExecutor : DefaultExecutorProvider.INSTANCE.get();
         this.ownedExecutor = providedExecutor == null && this.executor instanceof ExecutorService executorService
                 ? executorService
                 : null;

src/main/java/com/github/copilot/sdk/CopilotClient.java

@@ -7,12 +7,12 @@
 import java.util.concurrent.Executor;
 import java.util.concurrent.ForkJoinPool;
 
-final class DefaultExecutorProvider {
+enum DefaultExecutorProvider {
+    INSTANCE;
 
-    private DefaultExecutorProvider() {
-    }
+    private final Executor executor = ForkJoinPool.commonPool();
 
-    static Executor create() {
-        return ForkJoinPool.commonPool();
+    Executor get() {
+        return executor;
     }
 }

src/main/java/com/github/copilot/sdk/DefaultExecutorProvider.java

@@ -7,12 +7,12 @@
 import java.util.concurrent.Executor;
 import java.util.concurrent.Executors;
 
-final class DefaultExecutorProvider {
+enum DefaultExecutorProvider {
+    INSTANCE;
 
-    private DefaultExecutorProvider() {
-    }
+    private final Executor executor = Executors.newVirtualThreadPerTaskExecutor();
 
-    static Executor create() {
-        return Executors.newVirtualThreadPerTaskExecutor();
+    Executor get() {
+        return executor;
     }
 }

src/main/java25/com/github/copilot/sdk/DefaultExecutorProvider.java

@@ -4,11 +4,12 @@
 
 package com.github.copilot.sdk;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
-import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.io.IOException;
+import java.lang.reflect.Field;
 import java.lang.reflect.Method;
 import java.net.URL;
 import java.net.URLClassLoader;
@@ -17,6 +18,7 @@
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.Executor;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.ForkJoinPool;
 import java.util.concurrent.TimeUnit;
 import java.util.jar.Attributes;
 import java.util.jar.JarEntry;
@@ -28,12 +30,10 @@
 class DefaultExecutorProviderTest {
 
     @Test
-    void baseProviderUsesCompletableFutureDefaultExecutor() {
-        if (Runtime.version().feature() >= 25) {
-            return;
-        }
-
-        assertNull(DefaultExecutorProvider.create());
+    void testBaseImplementationReturnsForkJoinPool() {
+        Executor executor = DefaultExecutorProvider.INSTANCE.get();
+        assertNotNull(executor);
+        assertEquals(ForkJoinPool.commonPool(), executor);
     }
 
     @Test
@@ -54,10 +54,14 @@ void multiReleaseProviderUsesVirtualThreadsOnJdk25() throws Exception {
 
             try (var loader = new URLClassLoader(new URL[]{jar.toUri().toURL()}, null)) {
                 Class<?> provider = Class.forName("com.github.copilot.sdk.DefaultExecutorProvider", true, loader);
-                Method create = provider.getDeclaredMethod("create");
-                create.setAccessible(true);
-
-                Object result = create.invoke(null);
+                Field instanceField = provider.getDeclaredField("INSTANCE");
+                instanceField.setAccessible(true);
+                Object instance = instanceField.get(null);
+                
+                Method getMethod = provider.getDeclaredMethod("get");
+                getMethod.setAccessible(true);
+                Object result = getMethod.invoke(instance);
+                
                 assertNotNull(result, "JDK 25 multi-release provider must create a default executor");
                 assertTrue(result instanceof Executor, "Default provider must return an Executor");