Ask for JSON when autocompleting people to ping

`as: "json"` is a @rails/request.js option; plain fetch ignores it and
sends `Accept: */*`, so Autocompletable::UsersController answered with
its HTML format and response.json() threw. The new ping form never
showed any suggestions. Send an explicit Accept header instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/system/starting_pings_test.rb

@@ -0,0 +1,15 @@
+require "application_system_test_case"
+
+class StartingPingsTest < ApplicationSystemTestCase
+  setup do
+    sign_in "kevin@37signals.com"
+  end
+
+  test "suggesting people to ping" do
+    visit new_rooms_direct_url
+
+    find("[data-autocomplete-target='input']").send_keys("Jas")
+
+    assert_selector "suggestion-option[role='option']", text: "Jason"
+  end
+end