fix: Prevent CC/BCC field reset on chat activity actions (#11342)

# Pull Request Template

## Description

This PR fixes the CC/BCC field reset issue on activity action.
Fixes
[CW-4256](https://linear.app/chatwoot/issue/CW-4256/emails-added-in-cc-and-bcc-disappears-when-you-click-on-assign-to-me),
#5234

### Cause of the Issue
Previously, the CC and BCC fields in the reply box were being reset
whenever a conversation activity occurred (such as assignment, status
change, etc.). This happened because watchers on the current chat
messages array and current chat object would trigger
`setCCAndToEmailsFromLastChat` even when the last message was an
activity or system message, not a real email.

### Solution

- The updated logic ensures that the CC and BCC fields are only set
under the following conditions:
   **1**. Switching to a new conversation.
**2**. In the same conversation, only if the last message is not an
activity (i.e., only for actual emails).

- It uses the `lastEmail` computed property, which filters out private
and activity messages, ensuring only real email context changes update
the fields.
- This prevents user-entered `CC/BCC` values from being cleared after
system events (e.g., assignments or status changes), while still
updating the fields correctly when relevant messages are received.

## Type of change

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

## How Has This Been Tested?

### Loom video
**Before**

https://www.loom.com/share/2ec50dd1c0ed4eaf9170465274bea41e?sid=cc8b88cb-fd39-473a-8df3-78a242c8407b

**After**

https://www.loom.com/share/17fd2d96d5d84a049dcbf20d401d2ada?sid=8949ad48-7769-49d2-92c5-267da8c60d6e



## 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
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/javascript/dashboard/components/widgets/conversation/ReplyBox.vue

@@ -388,9 +388,14 @@ export default {
     },
   },
   watch: {
-    currentChat(conversation) {
+    currentChat(conversation, oldConversation) {
       const { can_reply: canReply } = conversation;
-      this.setCCAndToEmailsFromLastChat();
+      if (oldConversation && oldConversation.id !== conversation.id) {
+        // Only update email fields when switching to a completely different conversation (by ID)
+        // This prevents overwriting user input (e.g., CC/BCC fields) when performing actions
+        // like self-assign or other updates that do not actually change the conversation context
+        this.setCCAndToEmailsFromLastChat();
+      }
 
       if (this.isOnPrivateNote) {
         return;
@@ -406,13 +411,12 @@ export default {
     },
     // When moving from one conversation to another, the store may not have the
     // list of all the messages. A fetch is subsequently made to get the messages.
-    // However, this update does not trigger the `currentChat` watcher.
-    // We can add a deep watcher to it, but then, that would be too broad of a net to cast
-    // And would impact performance too. So we watch the messages directly.
-    // The watcher here is `deep` too, because the messages array is mutated and
-    // not replaced. So, a shallow watcher would not catch the change.
-    'currentChat.messages': {
-      handler() {
+    // This watcher handles two main cases:
+    // 1. When switching conversations and messages are fetched/updated, ensures CC/BCC fields are set from the latest OUTGOING/INCOMING email (not activity/private messages).
+    // 2. Fixes and issue where CC/BCC fields could be reset/lost after assignment/activity actions or message mutations that did not represent a true email context change.
+    lastEmail: {
+      handler(lastEmail) {
+        if (!lastEmail) return;
         this.setCCAndToEmailsFromLastChat();
       },
       deep: true,