Fix async modifier on abstract methods

Only add async modifier when method is not abstract and returns Task-like type

Co-authored-by: CyrusNajmabadi <4564579+CyrusNajmabadi@users.noreply.github.com>

Author: Copilot

src/Analyzers/CSharp/Tests/GenerateMethod/GenerateMethodTests.cs

@@ -10613,4 +10613,33 @@ private int Goo()
                 }
             }
             """, new(parseOptions: CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp14)));
+
+    [Fact]
+    public Task TestGenerateAbstractMethodReturningTask()
+        => TestInRegularAndScriptAsync(
+            """
+            using System.Threading.Tasks;
+
+            abstract class C
+            {
+                async Task M()
+                {
+                    await [|M2|]();
+                }
+            }
+            """,
+            """
+            using System.Threading.Tasks;
+
+            abstract class C
+            {
+                async Task M()
+                {
+                    await M2();
+                }
+
+                protected abstract Task M2();
+            }
+            """,
+            index: 1);
 }

src/Analyzers/Core/CodeFixes/GenerateParameterizedMember/AbstractGenerateParameterizedMemberService.SignatureInfo.cs

@@ -105,7 +105,7 @@ public async ValueTask<IMethodSymbol> GenerateMethodAsync(
                 attributes: default,
                 accessibility: DetermineAccessibility(isAbstract),
                 modifiers: DeclarationModifiers.None
-                    .WithIsStatic(State.IsStatic).WithIsAbstract(isAbstract).WithIsUnsafe(isUnsafe).WithAsync(knownTypes.IsTaskLike(returnType)),
+                    .WithIsStatic(State.IsStatic).WithIsAbstract(isAbstract).WithIsUnsafe(isUnsafe).WithAsync(!isAbstract && knownTypes.IsTaskLike(returnType)),
                 returnType: returnType,
                 refKind: DetermineRefKind(cancellationToken),
                 explicitInterfaceImplementations: default,