FIX: Include 'notify staff' separator in chat message flag modal (#23301)

When flagging a chat message, and options included both notifying user and notifying staff, the modal was missing the separating text. This was happening because the #staffFlagsAvailable method was based on post flags, and the model for chat flags is slightly different. This fixes that by delegating to the relevant flag target object.

app/assets/javascripts/discourse/app/components/flag-action-type.hbs

@@ -31,10 +31,8 @@
       </div>
     </label>
   </div>
-  {{#if @includeSeparator}}
-    <hr />
-  {{/if}}
   {{#if this.staffFlagsAvailable}}
+    <hr />
     <h3>{{i18n "flagging.notify_staff"}}</h3>
   {{/if}}
 {{else}}

app/assets/javascripts/discourse/app/components/modal/flag.hbs

@@ -21,7 +21,6 @@
           @username={{@model.flagModel.username}}
           @staffFlagsAvailable={{this.staffFlagsAvailable}}
           @changePostActionType={{this.changePostActionType}}
-          @includeSeparator={{this.includeSeparator}}
         />
       </FlagSelection>
     </form>

app/assets/javascripts/discourse/app/components/modal/flag.js

@@ -7,7 +7,7 @@ import { MAX_MESSAGE_LENGTH } from "discourse/models/post-action-type";
 import User from "discourse/models/user";
 import { reload } from "discourse/helpers/page-reloader";
 
-const NOTIFY_MODERATORS_ID = 7;
+const NOTIFY_MODERATORS_KEY = "notify_moderators";
 
 export default class Flag extends Component {
   @service adminTools;
@@ -75,13 +75,6 @@ export default class Flag extends Component {
     return this.args.model.flagTarget.submitLabel();
   }
 
-  get includeSeparator() {
-    return (
-      this.staffFlagsAvailable ||
-      this.args.model.flagTarget.includeSeparator?.()
-    );
-  }
-
   get title() {
     return this.args.model.flagTarget.title();
   }
@@ -91,7 +84,7 @@ export default class Flag extends Component {
   }
 
   get staffFlagsAvailable() {
-    return this.args.model.flagModel.flagsAvailable?.length > 1;
+    return this.flagsAvailable.length > 1;
   }
 
   get submitEnabled() {
@@ -111,7 +104,7 @@ export default class Flag extends Component {
   }
 
   get notifyModeratorsFlag() {
-    return this.flagsAvailable.find((f) => f.id === NOTIFY_MODERATORS_ID);
+    return this.flagsAvailable.find((f) => f.id === NOTIFY_MODERATORS_KEY);
   }
 
   get canTakeAction() {

plugins/chat/assets/javascripts/discourse/lib/chat-message-flag.js

@@ -24,10 +24,6 @@ export default class ChatMessageFlag {
     return false;
   }
 
-  includeSeparator() {
-    return true;
-  }
-
   _rewriteFlagDescriptions(flags) {
     return flags.map((flag) => {
       flag.set(