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/jobs/conversations/resolution_job_spec.rb

@@ -48,6 +48,21 @@ RSpec.describe Conversations::ResolutionJob do
     end
   end
 
+  # 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, muted? can crash
+  # trying to call blocked? on nil. Fixes # (issue).
+  it 'skips orphan conversations without a contact' do
+    account.update(auto_resolve_after: 14_400, auto_resolve_ignore_waiting: false) # 10 days in minutes
+    orphan_conversation = create(:conversation, account: account, last_activity_at: 13.days.ago, waiting_since: nil)
+    orphan_conversation.update_columns(contact_id: nil, contact_inbox_id: nil) # rubocop:disable Rails/SkipsModelValidations
+    resolvable_conversation = create(:conversation, account: account, last_activity_at: 13.days.ago, waiting_since: nil)
+
+    described_class.perform_now(account: account)
+
+    expect(orphan_conversation.reload.status).to eq('open')
+    expect(resolvable_conversation.reload.status).to eq('resolved')
+  end
+
   it 'adds a label after resolution' do
     account.update(auto_resolve_label: 'auto-resolved', auto_resolve_after: 14_400)
     conversation = create(:conversation, account: account, last_activity_at: 13.days.ago, waiting_since: 13.days.ago)