fix: handle missing email channels without raising errors

- Return nil from NewConversationStrategy when channel not found
- Update tests to expect nil instead of exceptions
- Align BCC test with graceful handling behavior

app/services/mailbox/conversation_finder_strategies/new_conversation_strategy.rb

@@ -8,9 +8,7 @@ class Mailbox::ConversationFinderStrategies::NewConversationStrategy < Mailbox::
   # It should be used as the last strategy in the chain to ensure emails are never dropped.
   def find
     channel = EmailChannelFinder.new(mail).perform
-
-    # Match old SupportMailbox behavior - raise error when no channel found
-    raise 'Email channel/inbox not found' if channel.nil?
+    return nil unless channel # No valid channel found
 
     @account = channel.account
     @inbox = channel.inbox

spec/mailboxes/reply_mailbox_spec.rb

@@ -687,7 +687,8 @@ RSpec.describe ReplyMailbox do
         bcc_mail.mail['to'] = nil
         bcc_mail.mail['bcc'] = 'care@example.com'
 
-        expect { described_class.receive bcc_mail }.to raise_error('Email channel/inbox not found')
+        described_class.receive bcc_mail
+        expect(conversation.present?).to be(false)
       end
     end
   end

spec/services/mailbox/conversation_finder_strategies/new_conversation_strategy_spec.rb

@@ -111,13 +111,13 @@ RSpec.describe Mailbox::ConversationFinderStrategies::NewConversationStrategy do
         mail.to = ['nonexistent@example.com']
       end
 
-      it 'raises an error' do
+      it 'returns nil' do
         strategy = described_class.new(mail)
 
         expect do
-          strategy.find
-        end.to raise_error('Email channel/inbox not found')
-          .and not_change(Conversation, :count)
+          result = strategy.find
+          expect(result).to be_nil
+        end.not_to change(Conversation, :count)
       end
     end