Have Task choose free ports for dev servers (#6145)

scripts/find-free-port.ps1

@@ -0,0 +1,41 @@
+# Prints one free TCP port per preferred port given as an argument.
+#
+# For each element of -Preferred, emits that port if it's free; otherwise
+# emits a random free port in 20000-49999. Probes by attempting to bind a
+# TcpListener on loopback. Tracks picks within this run so outputs are
+# guaranteed distinct from each other.
+param([int[]]$Preferred)
+
+$script:picked = @()
+
+function Test-PortFree {
+    param([int]$Port)
+    if ($script:picked -contains $Port) { return $false }
+    try {
+        $listener = [System.Net.Sockets.TcpListener]::new([System.Net.IPAddress]::Loopback, $Port)
+        $listener.Start()
+        $listener.Stop()
+        return $true
+    } catch {
+        return $false
+    }
+}
+
+function Get-RandomFreePort {
+    while ($true) {
+        $port = Get-Random -Minimum 20000 -Maximum 50000
+        if (Test-PortFree $port) { return $port }
+    }
+}
+
+foreach ($p in $Preferred) {
+    if (Test-PortFree $p) {
+        $script:picked += $p
+    } else {
+        $script:picked += Get-RandomFreePort
+    }
+}
+
+foreach ($p in $script:picked) {
+    [Console]::Out.Write("$p`n")
+}