Skip the attach and prefixed-completion E2E tests on Windows PowerShell

Un-skipping the entire Windows PowerShell E2E suite (now that the host-start
hang is fixed) gave us real WinPS coverage for the first time since #2318, but
it also surfaced two pre-existing WinPS-specific failures that have nothing to
do with host startup:

- `CanAttachScriptWithPathMappings` wedges on the in-box, cross-process
  `Debug-Runspace` attach handshake and rides whatever timeout we set. CI
  failed it at 10s, then again at exactly 30s after I bumped the budgets, so
  it genuinely never completes rather than merely running slow. This is the
  in-box attach deadlock tracked by #2323.
- `CanSendCompletionResolveWithModulePrefixRequestAsync` gets an empty
  completion list from the Windows PowerShell server for a prefix-imported
  command (it fails before any help assertion, so it's help-independent).
  That's the same "completion works in PS7 but not WinPS" family as #1355.

Because startup no longer hangs, we don't need #2318's discovery-time
`[SkippableFactOnWindowsPowerShell]` attribute anymore: an in-body
`Skip.If(IsWindowsPowerShell, ...)` runs after `InitializeAsync` (which now
starts the server fine) and skips before the broken code, so both tests skip
cleanly in ~1ms under `powershell.exe` instead of hanging or failing.

This also reverts my earlier timeout bump on `CanAttachScriptWithPathMappings`
(back to the 15s/10s/30s budgets from `main`) since the bigger budgets didn't
help and the test no longer runs on Windows PowerShell anyway. Everything else
stays un-skipped.

Drafted by Copilot (Claude Opus 4.8).

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

Author: andyleejordan

test/PowerShellEditorServices.Test.E2E/DebugAdapterProtocolMessageTests.cs

@@ -630,14 +630,15 @@ public async Task CanLaunchScriptWithNewChildAttachSessionAsJob()
             await terminatedTcs.Task;
         }
 
-        // Generous timeout: the attach handshake and subsequent debugging are
-        // much slower under Windows PowerShell on constrained CI runners.
-        [SkippableFact(Timeout = 60000)]
+        [SkippableFact(Timeout = 15000)]
         public async Task CanAttachScriptWithPathMappings()
         {
             Skip.If(PsesStdioLanguageServerProcessHost.RunningInConstrainedLanguageMode,
                 "Breakpoints can't be set in Constrained Language Mode.");
 
+            Skip.If(PsesStdioLanguageServerProcessHost.IsWindowsPowerShell,
+                "In-box Windows PowerShell wedges on the cross-process Debug-Runspace attach handshake (see #2323).");
+
             string[] logStatements = ["$PSCommandPath", "after breakpoint"];
 
             await RunWithAttachableProcess(logStatements, async (filePath, processId, runspaceId) =>
@@ -770,7 +771,7 @@ WinPS will always need this.
                         break
                     }
 
-                    if (((Get-Date) - $start).TotalSeconds -gt 30) {
+                    if (((Get-Date) - $start).TotalSeconds -gt 10) {
                         throw 'Timeout waiting for Debug-Runspace to be subscribed.'
                     }
 
@@ -809,9 +810,8 @@ WinPS will always need this.
 
             TaskCompletionSource<int> ridOutput = new();
 
-            // Task shouldn't take longer than 60 seconds to complete (it is
-            // much slower under Windows PowerShell on constrained CI runners).
-            using CancellationTokenSource debugTaskCts = new CancellationTokenSource(TimeSpan.FromSeconds(60));
+            // Task shouldn't take longer than 30 seconds to complete.
+            using CancellationTokenSource debugTaskCts = new CancellationTokenSource(TimeSpan.FromSeconds(30));
             using CancellationTokenRegistration _ = debugTaskCts.Token.Register(ridOutput.SetCanceled);
             using Process? psProc = Process.Start(psi);
             try

test/PowerShellEditorServices.Test.E2E/LanguageServerProtocolMessageTests.cs

@@ -1051,9 +1051,12 @@ public async Task CanSendCompletionAndCompletionResolveRequestAsync()
             Assert.Contains(testDescription, updatedCompletionItem.Documentation.String);
         }
 
-        [Fact]
+        [SkippableFact]
         public async Task CanSendCompletionResolveWithModulePrefixRequestAsync()
         {
+            Skip.If(PsesStdioLanguageServerProcessHost.IsWindowsPowerShell,
+                "Module-prefixed command completion returns no items on the Windows PowerShell server (see #1355).");
+
             await PsesLanguageClient
             .SendRequest(
                 "evaluate",