Restart WebSocket server on EnteredPlayMode when domain reload is disabled

Root cause: OnPlayModeStateChanged stops the server on ExitingEditMode and
relies entirely on a domain reload (OnAfterAssemblyReload) to bring it back
-- it never restarts the server on EnteredPlayMode itself. That assumption
holds when Enter Play Mode Options are off, but breaks completely under
"Reload Scene without Reload Domain" (Project Settings > Editor > Enter
Play Mode Options), a real, supported Unity profile some projects pick
specifically for faster iteration.

With no domain reload on either side of Play Mode, nothing ever restarts
the server -- it stays down for the entire Play session, with no way for a
client to even ask Unity to exit Play, since that request needs this same
server. Confirmed live: an MCP client got locked in Play mode with no way
out until the Editor was closed by hand.

Fix: also attempt a restart on EnteredPlayMode, guarded by !IsListening so
it's a no-op when a domain reload already handled it (the previous
behavior is unchanged for projects with domain reload enabled). Covers
both Enter Play Mode Options profiles with one change.