Fix 15-second close delay after dialog tests

Dialog subprocesses were forked from TestProcess, inheriting its
multiprocessing Queue objects and their process-shared POSIX semaphores
(_wlock). If a fork happened while the feeder thread held _wlock, the
child exited without releasing it, permanently blocking the feeder
thread on the next wacquire() and stalling Python's atexit _finalize_join
— causing test_proc.join() (no timeout) to hang the app for ~15 seconds.

Fix: use multiprocessing.get_context('spawn') for dialog subprocesses so
they start with a clean interpreter and inherit no semaphores or Queue
state. Also add a terminate/kill fallback timeout to test_proc.join() as
a safety net, and fix the missing return in JsonRpcConnection.is_alive().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/jrpc.py

@@ -228,7 +228,7 @@ class JsonRpcConnection:
         self.recv_thread.join()
 
     def is_alive(self):
-        self.recv_thread.is_alive()
+        return self.recv_thread.is_alive()
 
     def wait_ready(self, timeout=None):
         return self._event_ready.wait(timeout)