FIX: ensures `all_mentioned_user_ids` is not used as identifier (#21452)

When making the list of users to notify we set `all_mentioned_user_ids` key on the `to_notify` Hash.

This hash will be passed around until the actual moment where we send the notifications:

```ruby
identifier_text =
  case identifier_type
  when :here_mentions
    "@here"
  when :global_mentions
    "@all"
  when :direct_mentions
    ""
  else
    "@#{identifier_type}"
  end
```

As not found `all_mentioned_user_ids` would end up being sent as `@all_mentioned_user_ids` which is obviously incorrect.

This commit is a direct fix to the issue and will remove the key as soon as we have used it sooner up in the chain.

This bug was reproducible when doing this sequence of events:

- create a message with a direct mention: `@bob hi`
- edit this message into a global mention `@all hi`

plugins/chat/lib/chat/notifier.rb

@@ -95,7 +95,7 @@ module Chat
       notify_creator_of_inaccessible_mentions(to_notify)
       notify_mentioned_users(to_notify, already_notified_user_ids: already_notified_user_ids)
 
-      to_notify
+      to_notify.except(:all_mentioned_user_ids)
     end
 
     private

plugins/chat/spec/lib/chat/notifier_spec.rb

@@ -121,6 +121,23 @@ describe Chat::Notifier do
       include_examples "channel-wide mentions"
       include_examples "ensure only channel members are notified"
 
+      describe "editing a direct mention into a global mention" do
+        let(:mention) { "hello @#{user_2.username}!" }
+
+        it "doesn't return :all_mentioned_user_ids" do
+          msg = build_cooked_msg(mention, user_1)
+
+          Chat::MessageUpdater.update(
+            guardian: user_1.guardian,
+            chat_message: msg,
+            new_content: "hello @all",
+          )
+
+          to_notify = described_class.new(msg, msg.created_at).notify_edit
+          expect(to_notify[:all_mentioned_user_ids]).not_to be_present
+        end
+      end
+
       describe "users ignoring or muting the user creating the message" do
         it "does not send notifications to the user who is muting the acting user" do
           Fabricate(:muted_user, user: user_2, muted_user: user_1)