Skip the flaky child-attach-session E2E test on Windows PowerShell

Un-skipping the Windows PowerShell E2E suite surfaced one more attach-family
flake: `CanLaunchScriptWithNewChildAttachSession`. It passed on the prior CI run
(`a4e8a823e`) but timed out (`TaskCanceledException` at its 30s budget) on the
next (`25d9e58dd`) with no relevant code change in between, so it's genuinely
flaky on the slow in-box Windows PowerShell CI runners rather than broken.

The test runs `Start-DebugAttachSession` and waits for the server's
`startDebugging` reverse-request round-trip; on in-box Windows PowerShell that
round-trip is slow enough to intermittently miss the timeout. That's the same
in-box attach-E2E reliability bucket as #2323, and its two siblings are already
skipped there: `CanAttachScriptWithPathMappings` (the cross-process
`Debug-Runspace` wedge) and `CanLaunchScriptWithNewChildAttachSessionAsJob`
(no `ThreadJob` on Windows PowerShell). So skip this one on Windows PowerShell
too, keeping the rest of the now-un-skipped DAP suite running.

The flake only reproduces on the constrained CI runner, not on a fast dev box,
so I'm matching how the sibling attach tests are handled rather than chasing a
timeout bump I can't validate locally.

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

@@ -570,6 +570,9 @@ public async Task CanLaunchScriptWithNewChildAttachSession(
             Skip.If(PsesStdioLanguageServerProcessHost.RunningInConstrainedLanguageMode,
                 "PowerShellEditorServices.Command is not signed to run FLM in Constrained Language Mode.");
 
+            Skip.If(PsesStdioLanguageServerProcessHost.IsWindowsPowerShell,
+                "The Start-DebugAttachSession reverse-request round-trip is flaky on in-box Windows PowerShell CI runners (see #2323).");
+
             string script = NewTestFile($"Start-DebugAttachSession {paramString}");
 
             using CancellationTokenSource timeoutCts = new(30000);