fix(email): Allow inbox OAuth replies without global SMTP (#13820)

Email inbox replies now work for Google and Microsoft OAuth inboxes even
when the self-hosted instance does not have global SMTP configured. This
keeps agent replies working for email channels that already have valid
inbox-level delivery settings.

fixes: chatwoot/chatwoot#13118
closes: chatwoot/chatwoot#13118

## Why
Self-hosted email inbox replies were blocked by a global SMTP guard in
the `email_reply` path. For OAuth-backed email inboxes, outbound
delivery is configured at the inbox level, so the mailer returned early
and the reply flow failed before sending.

## What this change does
- Allows the `email_reply` path to proceed when the inbox has SMTP
configured
- Allows the `email_reply` path to proceed when the inbox has Google or
Microsoft OAuth delivery configured
- Renames the touched mailer helper predicates to `?` methods for
clarity

## Validation
- Configure a Google email inbox on a self-hosted instance without
global `SMTP_ADDRESS`
- Reply from Chatwoot to an existing email conversation
- Confirm the reply is sent through the inbox OAuth SMTP configuration
- Run `bundle exec rspec
spec/mailers/conversation_reply_mailer_spec.rb:595`

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

spec/mailers/conversation_reply_mailer_spec.rb

@@ -591,6 +591,30 @@ RSpec.describe ConversationReplyMailer do
         expect(mail.delivery_method.settings[:address]).to eq 'smtp.gmail.com'
         expect(mail.delivery_method.settings[:port]).to eq 587
       end
+
+      it 'uses inbox oauth smtp when global smtp config is unavailable' do
+        allow(class_instance).to receive(:smtp_config_set_or_development?).and_return(false)
+
+        mail = described_class.email_reply(message)
+
+        expect(mail).not_to be_nil
+        expect(mail.delivery_method.settings[:address]).to eq 'smtp.gmail.com'
+        expect(mail.delivery_method.settings[:port]).to eq 587
+      end
+    end
+
+    context 'when oauth provider is set but imap is disabled' do
+      let(:google_channel) do
+        create(:channel_email, imap_enabled: false, account: account, provider: 'google', provider_config: { access_token: 'access_token' })
+      end
+      let(:conversation) { create(:conversation, assignee: agent, inbox: google_channel.inbox, account: account).reload }
+      let(:message) { create(:message, conversation: conversation, account: account, message_type: 'outgoing', content: 'Outgoing Message 2') }
+
+      it 'does not build the mail without global smtp' do
+        allow(class_instance).to receive(:smtp_config_set_or_development?).and_return(false)
+
+        expect(described_class.email_reply(message).deliver_now).to be_nil
+      end
     end
 
     context 'when smtp disabled for email channel', :test do