fix(email): resolve reply recipients from the message being sent (#15194)

When an agent loops a new address into an ongoing email thread and then
adds a private note, the reply currently goes out with no Cc, and with
To falling back to the conversation contact. The newly added person
never receives the mail. This change makes an outgoing email keep the
To/Cc/Bcc that were entered on that reply, no matter what is added to
the conversation afterwards.

Closes #15193

## How to reproduce

1. Open a conversation on an Email inbox.
2. Add a new address to **Cc** and send a reply.
3. Immediately add a private note to the same conversation.
4. Inspect the delivered mail: `Cc` is empty and `To` is the
conversation contact instead of the addresses entered on the reply.

## What changed

`ConversationReplyMailer#cc_bcc_emails` and
`#to_emails_from_content_attributes` read the addresses from
`@conversation.messages.outgoing.last` rather than from the message they
were handed. Replies are delivered asynchronously
(`SendReplyJob.perform_later`, and `wait: 2.seconds` when the message
has attachments), so any outgoing message created in that window
replaces the recipients of a mail that is already queued. A private note
is the easiest way to hit it: private notes are outgoing messages, and
on an Email inbox `Messages::MessageBuilder#process_emails` stores them
with empty `to_emails` / `cc_emails` / `bcc_emails`.

Both lookups now go through the existing `current_message` helper
(`@message || @conversation.messages.outgoing.last`), which the `from`
and `reply_to` builders already use. `email_reply` sets `@message`, so
it resolves the recipients of the message being delivered;
`reply_with_summary` and `reply_without_summary` leave `@message` nil
and keep their current behaviour.

Added a spec covering the private-note case in
`spec/mailers/conversation_reply_mailer_spec.rb`.

## Note

I could not run the Ruby test suite locally — there is no Ruby toolchain
in the environment I worked in, so the new spec has not been executed on
my side. The change and the spec were verified by reading the code paths
(`Message#send_reply`, `Email::SendOnEmailService`,
`Messages::MessageBuilder#process_emails`,
`ConversationReplyMailerHelper#prepare_mail`). Deferring to CI for the
actual run.

app/mailers/conversation_reply_mailer.rb

@@ -174,7 +174,7 @@ class ConversationReplyMailer < ApplicationMailer
   end
 
   def cc_bcc_emails
-    content_attributes = @conversation.messages.outgoing.last&.content_attributes
+    content_attributes = current_message&.content_attributes
 
     return [] unless content_attributes
     return [] unless content_attributes[:cc_emails] || content_attributes[:bcc_emails]
@@ -183,7 +183,7 @@ class ConversationReplyMailer < ApplicationMailer
   end
 
   def to_emails_from_content_attributes
-    content_attributes = @conversation.messages.outgoing.last&.content_attributes
+    content_attributes = current_message&.content_attributes
 
     return [] unless content_attributes
     return [] unless content_attributes[:to_emails]

spec/mailers/conversation_reply_mailer_spec.rb

@@ -342,6 +342,25 @@ RSpec.describe ConversationReplyMailer do
         expect(mail.message_id).to eq("conversation/#{conversation.uuid}/messages/#{message.id}@#{conversation.account.domain}")
       end
 
+      context 'when a newer outgoing message exists in the conversation' do
+        let!(:message) do
+          create(:message, conversation: conversation, account: account, message_type: 'outgoing', content: 'Looping in the vendor',
+                           content_attributes: { to_emails: ['customer@example.com'], cc_emails: ['vendor@example.com'],
+                                                 bcc_emails: ['audit@example.com'] })
+        end
+
+        it 'sends to the recipients of the message being delivered' do
+          # a private note added right after the reply carries empty recipient lists
+          create(:message, conversation: conversation, account: account, message_type: 'outgoing', private: true,
+                           content: 'Vendor has been looped in',
+                           content_attributes: { to_emails: [], cc_emails: [], bcc_emails: [] })
+
+          expect(mail.to).to eq(message.content_attributes[:to_emails])
+          expect(mail.cc).to eq(message.content_attributes[:cc_emails])
+          expect(mail.bcc).to eq(message.content_attributes[:bcc_emails])
+        end
+      end
+
       context 'when message is a CSAT survey' do
         let(:csat_message) do
           create(:message, conversation: conversation, account: account, message_type: 'template',