Fix Windows service shutdown signalling (#999)

Stopping the zenserver Windows service (via `sc stop`, `zen service stop`, system shutdown, or any other SCM path) was being ignored. SCM would eventually force-kill the process after its timeout, giving an ungraceful shutdown.

## Root cause

PR #751 ("add simple http client tests", c37421a3b) restructured each HTTP server's `OnRun` loop from

```cpp
do { m_ShutdownEvent.Wait(WaitTimeout); }
while (!IsApplicationExitRequested());
```

to

```cpp
do { ShutdownRequested = m_ShutdownEvent.Wait(WaitTimeout); }
while (!ShutdownRequested);
```

That was well-intentioned — tests wanted to start/stop an HTTP server without touching global process state — but the old loop was the only thing that turned `RequestApplicationExit()` into an actual server wake-up. Once it was removed, `RequestApplicationExit(0)` was silently downgraded to "just sets a flag". The `WindowsService::SvcCtrlHandler` stop path was calling exactly that, so SCM stops stopped working. The sponsor-process check path kept working only because it *also* calls `m_Http->RequestExit()` via `ZenServerBase::RequestExit()`.

## Fix

- Restore `IsApplicationExitRequested()` as a secondary exit condition in each HTTP server's `OnRun` loop (`httpsys`, `httpasio`, `httpmulti`, `httpnull`, `httpplugin`) alongside the per-server `m_ShutdownEvent` that #751 introduced. Preserves #751's goal — tests can still call `server->RequestExit()` without touching global state — while making `RequestApplicationExit()` wake the server up again, which the rest of the codebase and `SvcCtrlHandler` assume.
- Clean up the service control handler in the same pass: also accept `SERVICE_CONTROL_SHUTDOWN`, report `STOP_PENDING` with a 30s `dwWaitHint` (was 0), drop the redundant second `ReportSvcStatus` call, and remove `ghSvcStopEvent` which nothing ever `Wait()`-ed on.
- Advertise `SERVICE_ACCEPT_STOP | SERVICE_ACCEPT_SHUTDOWN` while running; drop controls while stop-pending/stopped.
- Make `WindowsService` destructor virtual (latent UB given `Run()` was already virtual).

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