ci: try to fix playwright issue

"Navigation to X is interrupted to another navigation to Y"

In the logs we see:
```
FAILED tests/integration/ssg_test.py::test_ssg[starlette-chromium] - playwright._impl._errors.Error: Navigation to "http://localhost:18768/" is interrupted by another navigation to "http://localhost:18768/"
FAILED tests/integration/starlette_test.py::test_starlette_mount[starlette-chromium] - playwright._impl._errors.Error: Navigation to "http://localhost:18770/solara_mount/" is interrupted by another navigation to "http://localhost:18768/"
ERROR tests/integration/widget_test.py::test_widget_button_solara[starlette-chromium] - playwright._impl._errors.Error: Navigation to "http://localhost:18768/?id=4004e29b-a9e6-4b1e-9652-7b1c61ef86cd" is interrupted by another navigation to "http://localhost:18770/solara_mount/"
ERROR tests/integration/widget_test.py::test_solara_button_all[starlette-solara-chromium] - playwright._impl._errors.Error: Navigation to "http://localhost:18768/?id=c8f0b1fb-abdb-4969-b340-1156d2699783" is interrupted by another navigation to "http://localhost:18768/?id=4004e29b-a9e6-4b1e-9652-7b1c61ef86cd"
ERROR tests/integration/widget_test.py::test_slider_all[starlette-solara-chromium] - playwright._impl._errors.Error: Navigation to "http://localhost:18768/?id=680f1fc7-5a19-4e1c-a382-088f2ec96b78" is interrupted by another navigation to "http://localhost:18768/?id=c8f0b1fb-abdb-4969-b340-1156d2699783"
```

packages/solara-enterprise/solara_enterprise/ssg.py

@@ -49,6 +49,7 @@ def _get_playwright():
 
     pw.browser = pw.sync_playwright.chromium.launch(headless=not settings.ssg.headed)
     pw.page = pw.browser.new_page()
+    playwrights.append(pw)
     return pw
 
 
@@ -81,12 +82,13 @@ def wait(async_result):
 
     for result in results:
         wait(result)
-    thread_pool.terminate()
+    thread_pool.close()
+    thread_pool.join()
     for pw in playwrights:
         assert pw.browser is not None
         assert pw.context_manager is not None
         pw.browser.close()
-        pw.context_manager.stop()
+        pw.context_manager.__exit__(None, None, None)
 
     rprint("Done building SSG")