Wrap the Run() loop in try/finally so cleanup (_running = false;
ReleasePendingPrompt(null)) always executes, even if the injected
lookup throws. Previously a throwing lookup escaped the loop and left
any pending ReadLine() caller blocked forever. Also fold the lookup
call into the same try/catch as the handler invocation so a bad
command is swallowed without tearing down the reader loop.
Document via XML comments that ReadLine() supports a single sequential
caller (not concurrent callers) and that command handlers dispatched
by Run() execute on the reader thread and must not call ReadLine().
Add a regression test proving a throwing lookup neither hangs a
pending prompt nor kills the reader loop. Also drop an unused
System.Text using from the test file.
