feat: show email subject in conversation search results (#10843)

# Pull Request Template

## Description

This addresses #10842. It exposes `additional_attributes` in the
conversations search endpoint, uses it in
`SearchResultConversationsList` to pass
`conversation.additional_attributes?.mail_subject` down to
`SearchResultConversationItem`, which in turn displays it.

Fixes #10842

## Type of change

Please delete options that are not relevant.

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

## How Has This Been Tested?

I have tested this locally by searching for conversations. See this
screenshot where I searched for "noreply":
![Screenshot from 2025-02-05
11-04-54](https://github.com/user-attachments/assets/689e3e99-c20b-48a7-9c3e-35d45ffeafc1)

I would love to add automated tests but I’m not sure how to do that.

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] 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
- [x] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
Co-authored-by: iamsivin <iamsivin@gmail.com>

app/javascript/dashboard/i18n/locale/en/search.json

@@ -24,6 +24,7 @@
     "READ_MORE": "Read more",
     "WROTE": "wrote:",
     "FROM": "from",
-    "EMAIL": "email"
+    "EMAIL": "email",
+    "EMAIL_SUBJECT": "subject"
   }
 }