Fixed #36423 -- Prevented filter_horizontal buttons from intercepting form submission.

In the admin's filter_horizontal widget, optional action buttons like
"Choose all", "Remove all", etc. were changed from `<a>` to `<button>`
elements in #34619, but without specifying `type="button"`. As a result,
when pressing Enter while focused on a form input, these buttons could
be triggered and intercept form submission.

Explicitly set `type="button"` on these control buttons to prevent them
from acting as submit buttons.

Thanks Antoliny Lee for the quick triage and review.

Regression in 857b1048d5.

tests/admin_widgets/tests.py

@@ -1737,6 +1737,48 @@ class HorizontalVerticalFilterSeleniumTests(AdminWidgetSeleniumTestCase):
 
         self.assertCountSeleniumElements("#id_students_to > option", 2)
 
+    def test_form_submission_via_enter_key_with_filter_horizontal(self):
+        """
+        The main form can be submitted correctly by pressing the enter key.
+        There is no shadowing from other buttons inside the form.
+        """
+        from selenium.webdriver.common.by import By
+        from selenium.webdriver.common.keys import Keys
+
+        self.school.students.set([self.peter])
+        self.school.alumni.set([self.lisa])
+
+        self.admin_login(username="super", password="secret", login_url="/")
+        self.selenium.get(
+            self.live_server_url
+            + reverse("admin:admin_widgets_school_change", args=(self.school.id,))
+        )
+
+        self.wait_page_ready()
+        self.select_option("#id_students_from", str(self.lisa.id))
+        self.selenium.find_element(By.ID, "id_students_add").click()
+        self.select_option("#id_alumni_from", str(self.peter.id))
+        self.selenium.find_element(By.ID, "id_alumni_add").click()
+
+        # Trigger form submission via Enter key on a text input field.
+        name_input = self.selenium.find_element(By.ID, "id_name")
+        name_input.click()
+        name_input.send_keys(Keys.ENTER)
+
+        # Form was submitted, success message should be shown.
+        self.wait_for_text(
+            "li.success", "The school “School of Awesome” was changed successfully."
+        )
+
+        # Changes should be stored properly in the database.
+        school = School.objects.get(id=self.school.id)
+        self.assertSequenceEqual(
+            school.students.all().order_by("name"), [self.lisa, self.peter]
+        )
+        self.assertSequenceEqual(
+            school.alumni.all().order_by("name"), [self.lisa, self.peter]
+        )
+
 
 class AdminRawIdWidgetSeleniumTests(AdminWidgetSeleniumTestCase):
     def setUp(self):