fix: prevent NoMethodError in mute helpers when contact is nil (#13277)

## Linear Ticket

https://linear.app/chatwoot/issue/CW-4569/nomethoderror-undefined-method-blocked-for-nil-nomethoderror

## Description
Fixes NoMethodError in ConversationMuteHelpers that occurs during
contact deletion race condition.
When a contact is deleted, there's a brief window (~50-150ms) where
contact_id becomes nil but conversations still exist. If ResolutionJob
runs during this window, the muted? method crashes trying to call
blocked? on nil.Fixes # (issue)

## Type of change

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

## How Has This Been Tested?

- Created orphaned conversations (contact_id = nil)
- Called muted?, mute!, unmute! - all return gracefully
- Verified async deletion still works correctly

## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] 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
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] 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>

spec/models/conversation_spec.rb

@@ -390,6 +390,20 @@ RSpec.describe Conversation do
         .to(have_been_enqueued.at_least(:once).with(conversation, { account_id: conversation.account_id, inbox_id: conversation.inbox_id,
                                                                     message_type: :activity, content: "#{user.name} has muted the conversation" }))
     end
+
+    context 'when contact is missing' do
+      before do
+        conversation.update_columns(contact_id: nil, contact_inbox_id: nil) # rubocop:disable Rails/SkipsModelValidations
+      end
+
+      it 'does not change conversation status' do
+        expect { mute! }.not_to(change { conversation.reload.status })
+      end
+
+      it 'does not enqueue an activity message' do
+        expect { mute! }.not_to have_enqueued_job(Conversations::ActivityMessageJob)
+      end
+    end
   end
 
   describe '#unmute!' do
@@ -418,6 +432,22 @@ RSpec.describe Conversation do
         .to(have_been_enqueued.at_least(:once).with(conversation, { account_id: conversation.account_id, inbox_id: conversation.inbox_id,
                                                                     message_type: :activity, content: "#{user.name} has unmuted the conversation" }))
     end
+
+    context 'when contact is missing' do
+      let(:conversation) { create(:conversation) }
+
+      before do
+        conversation.update_columns(contact_id: nil, contact_inbox_id: nil) # rubocop:disable Rails/SkipsModelValidations
+      end
+
+      it 'does not change conversation status' do
+        expect { unmute! }.not_to(change { conversation.reload.status })
+      end
+
+      it 'does not enqueue an activity message' do
+        expect { unmute! }.not_to have_enqueued_job(Conversations::ActivityMessageJob)
+      end
+    end
   end
 
   describe '#muted?' do
@@ -433,6 +463,16 @@ RSpec.describe Conversation do
     it 'returns false if conversation is not muted' do
       expect(muted?).to be(false)
     end
+
+    context 'when contact is missing' do
+      before do
+        conversation.update_columns(contact_id: nil, contact_inbox_id: nil) # rubocop:disable Rails/SkipsModelValidations
+      end
+
+      it 'returns false' do
+        expect(muted?).to be(false)
+      end
+    end
   end
 
   describe 'unread_messages' do