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

scripts/find-free-port.ps1

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+# 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")
+}

scripts/find-free-port.sh

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+#!/usr/bin/env bash
+# Prints one free TCP port per preferred port given as an argument.
+#
+# For each argument, emits that port if it's free; otherwise emits a random
+# free port in 20000-49999. Probes via bash's /dev/tcp pseudo-device (connect
+# failure = nobody listening). Tracks picks within this run so outputs are
+# guaranteed distinct from each other.
+set -euo pipefail
+
+declare -a picked=()
+
+is_free() {
+  local port=$1
+  for p in ${picked[@]+"${picked[@]}"}; do
+    if [ "$p" = "$port" ]; then return 1; fi
+  done
+  if (exec 3<>"/dev/tcp/127.0.0.1/$port") >/dev/null 2>&1; then
+    return 1
+  fi
+  return 0
+}
+
+random_free_port() {
+  while true; do
+    local port=$((RANDOM % 30000 + 20000))
+    if is_free "$port"; then
+      echo "$port"
+      return
+    fi
+  done
+}
+
+for preferred in "$@"; do
+  if is_free "$preferred"; then
+    picked+=("$preferred")
+  else
+    picked+=("$(random_free_port)")
+  fi
+done
+
+for p in "${picked[@]}"; do
+  echo "$p"
+done