fix(console): stop headless servers from pegging a CPU core (#2535)

## Problem

On headless Linux deployments (systemd service, Docker without a TTY, `nohup`), the ModernUO process pegs a full CPU core even when idle. It does not reproduce on Windows because that runs with an interactive console.

## Root cause

`ConsoleInputHandler` runs a background thread (named "Console Input Handler") that loops on `Console.ReadLine()`. When stdin is **not** an interactive terminal, `Console.ReadLine()` returns `null` at end-of-stream **immediately** on every call, so the loop `continue`s in a tight spin — one core at 100%.

Reproduced in a container running the actual distribution: the "Console Input Handler" thread sat at ~90% CPU on a headless boot; with a blocking stdin it dropped to idle.

## Fix

1. **Detect headless once at startup:** `Core.Headless = Console.IsInputRedirected`.
2. **Extract a testable `ConsoleInputPump`** that owns the input stream: per line read, it *atomically* (under one lock) either delivers the line to a waiting prompt or dispatches a console command, and it **ends on EOF instead of spinning**. Cleanup runs unconditionally in a `finally`, so a pending prompt is always released (never hangs). Replaces the old `async void` loop and the fragile `_expectUserInput` / two-`AutoResetEvent` / `_input` handshake.
3. **`ConsoleInputHandler` becomes a thin headless-aware facade** over the pump. Headless: the reader thread never starts (`Console input disabled (headless: stdin is not a TTY).`), and `ReadLine()` throws a fatal `HeadlessConsoleInputException`.
4. **Data-gating and first-boot prompts** (deserialization "delete bad types? y/n", save-conflict, config/expansion setup) now route through `ConsoleInputHandler.ReadLine()`, so a headless server crashes fatal with a clear message instead of reading `null` (previously an NRE or a silent wrong branch).

Design decision (model b): headless servers are expected to be supplied with configuration/save data (including the owner account); interactive prompts when headless are fatal by design.

## Testing

- New `ConsoleInputPumpTests` (5 tests): EOF ends the loop without spinning; command dispatch; a pending prompt receives the next line; EOF while a prompt is pending completes it with `null` (no hang); a throwing command lookup does not hang a pending prompt. The tests synchronize on real pump state (no `Thread.Sleep`), so they are deterministic on slow CI.
- Full `Server.Tests`: no new failures introduced.

## End-to-end verification (Docker, real distribution)

| | Console Input Handler thread | Container CPU |
|---|---|---|
| Before fix (headless boot) | ~90% | ~199% (2 cores) |
| After fix (headless boot) | **not started** | **~11%** |

After the fix, a headless boot logs `Console input disabled (headless: stdin is not a TTY).`, loads the world normally, and idles instead of spinning.

Projects/Server/Configuration/ServerConfigurationPrompts.cs

@@ -16,7 +16,7 @@ public static class ServerConfigurationPrompts
         do
         {
             Console.Write("{0}> ", directories.Count > 0 ? "[enter to finish]" : " ");
-            var directory = Console.ReadLine();
+            var directory = ConsoleInputHandler.ReadLine();
             if (string.IsNullOrWhiteSpace(directory))
             {
                 break;
@@ -55,7 +55,7 @@ public static class ServerConfigurationPrompts
         {
             // IP:Port?
             Console.Write("[{0}]> ", ips.Count > 0 ? "enter to finish" : "0.0.0.0:2593");
-            var ipStr = Console.ReadLine();
+            var ipStr = ConsoleInputHandler.ReadLine();
 
             IPEndPoint ip;
             if (string.IsNullOrWhiteSpace(ipStr))
@@ -102,7 +102,7 @@ public static class ServerConfigurationPrompts
         do
         {
             Console.Write("[ModernUO]> ");
-            serverName = Console.ReadLine();
+            serverName = ConsoleInputHandler.ReadLine();
 
             if (string.IsNullOrWhiteSpace(serverName))
             {