fix: undo no longer fails with a 400 in a signed-in browser

Undo ends every session of the user, including the one that opened the link; saving that deleted session at the end of the request made Django answer 400 although the address had been restored. The view now drops its own session (as sign-out does), also on the unclear-outcome page. Tests for both cases from a signed-in browser.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FocPJ75XPma7TcuYEMy8zm

portal/tests/test_email_change.py

@@ -440,6 +440,19 @@ def test_undoing_restores_the_address_ends_the_sessions_and_tells_the_admin(peop
     assert Client().post(path).status_code == 410 and Client().get(path).status_code == 410
 
 
+def test_undoing_from_a_browser_that_is_still_signed_in_works(people, adapters):
+    path = confirmed_undo_path(adapters)
+    browser = signed_in(adapters)  # the link is opened where the user is signed in
+    response = browser.post(path)
+    assert (
+        response.status_code == 200
+        and "Please change your password now." in response.content.decode()
+    )
+    assert mail_of(adapters) == OLD
+    assert browser.get("/").status_code == 302  # its own session ended too
+    assert UserSession.objects.filter(username="alice").count() == 0
+
+
 def test_undoing_also_drops_a_request_made_in_the_meantime(people, adapters):
     path = confirmed_undo_path(adapters)
     signed_in(adapters)
@@ -593,6 +606,20 @@ def test_a_failure_after_the_undoing_write_still_ends_the_sessions(
     assert "user=alice" in caplog.text and OLD not in caplog.text
 
 
+def test_an_unclear_undo_from_a_signed_in_browser_shows_the_unclear_page(
+    people, adapters, monkeypatch
+):
+    path = confirmed_undo_path(adapters)
+    browser = signed_in(adapters)
+
+    def broken(*args, **kwargs):
+        raise RuntimeError("database trouble")
+
+    monkeypatch.setattr(services.mailer, "enqueue", broken)
+    assert browser.post(path).status_code == 500  # not a 400 from its own ended session
+    assert browser.get("/").status_code == 302
+
+
 def test_a_failure_before_any_write_leaves_the_request_clean(people, adapters, monkeypatch):
     client = signed_in(adapters)