node:http: defer server 'close' until every tracked connection has ended - #35837

Deferring `'close'` until `kTrackedConnections` drains is correct, but `close()` sets `this[serverSymbol] = undefined` (line 523), which makes `closeAllConnections()` (line 488) and `closeIdleConnections()` (line 510) no-ops. So the documented Node deadline pattern — `server.close(cb); setTimeout(() => server.closeAllConnections(), N)` — now hangs where pre-PR it (incorrectly, but terminatingly) fired `cb` early. When `serverSymbol` is already cleared, both methods should fall back to iterating 

`kPendingDrainClose` is set to `true` here but never reset in `listen()`/`kRealListen`, and `emitCloseServer` doesn't check `self[serverSymbol]`. If the user re-listens while the flag is armed, the old keep-alive connection's `#onClose` will schedule `emitCloseServer` and fire `'close'` (plus the stored close callback) on an actively-listening server. Node's `_emitCloseIfDrained` — which the comment says this mirrors — guards this with `if (this._handle || this._connections) return`; add the equ