fix(contacts): Show telegram id in contact details form (#13611)

## Summary
This change fixes a mismatch in contact details where Telegram data
could be shown in the contact profile/social icon area but was not
available in the editable contact form.

### What changed
- Added Telegram to the social links section of the next-gen contact
form so agents can view and edit it alongside Facebook, Instagram,
TikTok, Twitter, GitHub, and LinkedIn.
- Added Telegram support to the legacy conversation contact edit form
for parity between both contact editing experiences.
- Mapped social_telegram_user_name into the editable socialProfiles
payload when preparing contact form state, so Telegram usernames sourced
from channel attributes are visible in the form.
- Updated the conversation contact social profile merge logic so
Telegram display prefers an explicitly saved social profile value and
falls back to social_telegram_user_name when needed.
- Added the missing English i18n placeholder: Add Telegram.

### Why
Without this, users could see Telegram info in some contact views but
could not reliably edit it in contact details, creating inconsistent
behavior between display and edit states.

---------

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

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

@@ -458,6 +458,9 @@
           "INSTAGRAM": {
             "PLACEHOLDER": "Add Instagram"
           },
+          "TELEGRAM": {
+            "PLACEHOLDER": "Add Telegram"
+          },
           "TIKTOK": {
             "PLACEHOLDER": "Add TikTok"
           },