chore(diagnostic): log exception type + stack trace when polling-engine outer catch fires

Captures Exception.GetType().FullName, Message, ScriptStackTrace,
.NET StackTrace, and InnerException details to
%TEMP%/mcp-polling-engine-error.log on each outer-catch hit.

Triggered by an intermittent "Operation is not valid due to the
current state of the object" the user saw whose source isn't yet
identified. Without a stack trace we can only guess between
Format-McpOutput, Get-Location, the Console.SetOut/SetError restore
sequence, the _externalUI reflection swap, or PSReadLine timing.
The log will pin it to the failing line on next reproduction.

Wrapped in try/catch so any failure of the diagnostic itself (file
I/O, Add-Content lock contention) cannot mask or replace the
original exception that brought us into this catch.

REMOVE this block once the underlying cause has been identified and
fixed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yotsuda

PowerShell.MCP/Resources/MCPPollingEngine.ps1

@@ -895,6 +895,41 @@ if (-not (Test-Path Variable:global:McpTimer)) {
                 catch {
                     $errorMessage = "Command execution failed: $($_.Exception.Message)"
                     Write-Host $errorMessage -ForegroundColor Red
+                    # Diagnostic: capture stack trace + exception type so we can
+                    # locate the failing code path the next time this fires.
+                    # Triggered by an intermittent "Operation is not valid due
+                    # to the current state of the object" the user saw whose
+                    # source isn't yet identified — remove this block once the
+                    # underlying cause has been pinned down.
+                    try {
+                        $diagPath = Join-Path $env:TEMP 'mcp-polling-engine-error.log'
+                        $stamp = Get-Date -Format 'yyyy-MM-ddTHH:mm:ss.fff'
+                        $cmdSnippet = if ($cmd) { $cmd.Substring(0, [Math]::Min(200, $cmd.Length)) } else { '<null>' }
+                        $exType = $_.Exception.GetType().FullName
+                        $exMsg  = $_.Exception.Message
+                        $scriptST = if ($_.ScriptStackTrace) { $_.ScriptStackTrace } else { '<null>' }
+                        $netST    = if ($_.Exception.StackTrace) { $_.Exception.StackTrace } else { '<null>' }
+                        $inner = $_.Exception.InnerException
+                        $innerType = if ($inner) { $inner.GetType().FullName } else { '<none>' }
+                        $innerMsg  = if ($inner) { $inner.Message } else { '' }
+                        $innerST   = if ($inner -and $inner.StackTrace) { $inner.StackTrace } else { '' }
+                        $diagText = @"
+===== $stamp =====
+Pipeline (truncated): $cmdSnippet
+Exception type: $exType
+Message: $exMsg
+ScriptStackTrace:
+$scriptST
+.NET StackTrace:
+$netST
+InnerException type: $innerType
+InnerException message: $innerMsg
+InnerException StackTrace:
+$innerST
+
+"@
+                        Add-Content -Path $diagPath -Value $diagText -Encoding UTF8
+                    } catch {}
                     $mcpOutput = $errorMessage
                 }
                 finally {