fix: Enhance notification emails with message details and handle failed messages (#13273)

## Description

Handle messages with null content properly in UI and email notifications

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)

## Relevant Screenshots:
<img width="688" height="765" alt="Screenshot 2026-01-21 at 4 43 00 PM"
src="https://github.com/user-attachments/assets/6a27c22e-2ae6-4377-a05d-cfa44bf181fe"
/>


## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches notification email templates and message rendering conditions;
mistakes could lead to missing content/attachments in emails or
incorrect UI visibility, but changes are localized and non-auth/security
related.
> 
> **Overview**
> Agent notification emails for *assigned* and *participating* new
messages now include the actual message details (sender name, rendered
text when present, and attachment links) and gracefully fall back when
content is unavailable.
> 
> To support this, the mailer now passes `@message` into Liquid via
`MessageDrop` (adding `attachments` URLs), and the dashboard message UI
now renders failed/external-error messages even when `content` is `null`
while tightening retry eligibility to require content or attachments
(and still within 1 day).
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
475c8cedda54eb5e806990f977faf8098d0b27d8. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/javascript/dashboard/components-next/message/Message.vue

@@ -391,13 +391,17 @@ const shouldRenderMessage = computed(() => {
   const isUnsupported = props.contentAttributes?.isUnsupported;
   const isAnIntegrationMessage =
     props.contentType === CONTENT_TYPES.INTEGRATIONS;
+  const isFailedMessage = props.status === MESSAGE_STATUS.FAILED;
+  const hasExternalError = !!props.contentAttributes?.externalError;
 
   return (
     hasAttachments ||
     props.content ||
     isEmailContentType ||
     isUnsupported ||
-    isAnIntegrationMessage
+    isAnIntegrationMessage ||
+    isFailedMessage ||
+    hasExternalError
   );
 });
 

app/javascript/dashboard/components-next/message/MessageError.vue

@@ -12,11 +12,16 @@ defineProps({
 
 const emit = defineEmits(['retry']);
 
-const { orientation, status, createdAt } = useMessageContext();
+const { orientation, status, createdAt, content, attachments } =
+  useMessageContext();
 
 const { t } = useI18n();
 
-const canRetry = computed(() => !hasOneDayPassed(createdAt.value));
+const canRetry = computed(() => {
+  const hasContent = content.value !== null;
+  const hasAttachments = attachments.value && attachments.value.length > 0;
+  return !hasOneDayPassed(createdAt.value) && (hasContent || hasAttachments);
+});
 </script>
 
 <template>