fix(new-conversation): stop writing literal "undefined" as mail_subject (#14383)

## Description

This PR fixes a data-corruption bug in the new conversation flow that
surfaces in conversation search results as `Subject: undefined`.

### The Problem
When creating a conversation through the "New conversation" modal
without typing a subject (every non-email channel never shows the
subject input, and email channels can be left blank), the conversation
gets persisted with `additional_attributes.mail_subject = "undefined"`
(the literal string). The bogus value shows up in conversation search
results as `Subject: undefined`, which started rendering after #10843.

### Root Cause
`createConversationPayload` in
`app/javascript/dashboard/store/modules/contactConversations.js` appends
the `mail_subject` field unconditionally:

```js
payload.append('additional_attributes[mail_subject]', mailSubject);
```

`composeConversationHelper.js` only sets `payload.mailSubject` when
`subject` is truthy, so when no subject is provided, `mailSubject` is
destructured as `undefined` in `createConversationPayload`.
`FormData.append` coerces `undefined` to the string `"undefined"`, and
the backend persists it as-is into the JSONB column.

### The Fix
Skip the append when `mailSubject` is falsy. The backend already treats
a missing key the same as an empty subject (the email mailer falls back
to a default subject when `mail_subject` is `nil`), so omitting the
field is safe across channels.

### Key Changes
- Guarded the `additional_attributes[mail_subject]` append in
`createConversationPayload`.
- Added a spec covering the case where `mailSubject` is omitted.

> Note: existing rows persisted before this fix will continue to show
`Subject: undefined` in search until cleaned up. A simple SQL cleanup
for non-email inboxes:
>
> ```sql
> UPDATE conversations
> SET additional_attributes = additional_attributes - 'mail_subject'
> FROM inboxes
> WHERE conversations.inbox_id = inboxes.id
>   AND inboxes.channel_type <> 'Channel::Email'
> AND conversations.additional_attributes->>'mail_subject' =
'undefined';
> ```
>
> I'm leaving any data-cleanup migration out of this PR since it's a
maintenance concern that may be handled differently per installation.

## Type of change

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

## How Has This Been Tested?

1. Open the "New conversation" modal on a non-email inbox (e.g.,
WhatsApp, SMS, API).
2. Create a conversation without filling any subject (field is not
present on those, so regular flow).
3. Open the global search and search for the conversation.
4. **Before:** the result card shows `Subject: undefined`.
   **After:** the subject row is hidden.
5. Repeat on an email inbox leaving the subject blank — same result.
6. On an email inbox, type a subject and verify it still persists and
renders correctly.
7. Run the new spec: `pnpm test contactConversations`.

## Checklist:

- [X] My code follows the style guidelines of this project
- [X] 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
- [X] My changes generate no new warnings
- [X] I have added tests that prove my fix is effective or that my
feature works
- [X] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

Co-authored-by: Sojan Jose <sojan@pepalo.com>
Co-authored-by: Sony Mathew <sony@chatwoot.com>

app/javascript/dashboard/store/modules/contactConversations.js

@@ -25,7 +25,9 @@ export const createConversationPayload = ({ params, contactId, files }) => {
   payload.append('inbox_id', inboxId);
   payload.append('contact_id', contactId);
   payload.append('source_id', sourceId);
-  payload.append('additional_attributes[mail_subject]', mailSubject);
+  if (mailSubject) {
+    payload.append('additional_attributes[mail_subject]', mailSubject);
+  }
   payload.append('assignee_id', assigneeId);
 
   return payload;

app/javascript/dashboard/store/modules/specs/contactConversations/actions.spec.js

@@ -282,6 +282,24 @@ describe('createConversationPayload', () => {
     expect(payload.get('assignee_id')).toBe(options.params.assigneeId);
     expect(payload.getAll('message[attachments][]')).toEqual([]);
   });
+
+  it('omits mail_subject when mailSubject is undefined', () => {
+    const options = {
+      params: {
+        inboxId: '1',
+        message: {
+          content: 'Test message content',
+        },
+        sourceId: '12',
+        assigneeId: '123',
+      },
+      contactId: '23',
+    };
+
+    const payload = createConversationPayload(options);
+
+    expect(payload.has('additional_attributes[mail_subject]')).toBe(false);
+  });
 });
 
 describe('createWhatsAppConversationPayload', () => {