fix(account-deletion): normalize deleted email suffix and handle collisions safely (#13472)

## Summary
This PR fixes account deletion failures by changing how orphaned user
emails are rewritten during `AccountDeletionService`.

Ref:
https://chatwoot-p3.sentry.io/issues/6715254765/events/e228a5d045ad47348d6c32448bc33b7a/

## Changes (develop -> this branch)
- Updated soft-delete email rewrite from:
  - `#{original_email}-deleted.com`
- To deterministic value:
  - `#{user.id}@chatwoot-deleted.invalid`
- Added reserved non-deliverable domain constant:
  - `@chatwoot-deleted.invalid`
- Replaced the "other accounts" check from `count.zero?` to `exists?`
(same behavior, cheaper query).
- Updated service spec expectation to match deterministic email value
and assert it differs from original email.

## Files changed
- `app/services/account_deletion_service.rb`
- `spec/services/account_deletion_service_spec.rb`

## How to verify
- Run: `bundle exec rspec
spec/services/account_deletion_service_spec.rb`
- Run: `bundle exec rubocop app/services/account_deletion_service.rb
spec/services/account_deletion_service_spec.rb`

spec/services/account_deletion_service_spec.rb

@@ -46,7 +46,8 @@ RSpec.describe AccountDeletionService do
 
         # Reload the user to get the updated email
         user_with_one_account.reload
-        expect(user_with_one_account.email).to eq("#{original_email}-deleted.com")
+        expect(user_with_one_account.email).to eq("#{user_with_one_account.id}@chatwoot-deleted.invalid")
+        expect(user_with_one_account.email).not_to eq(original_email)
       end
 
       it 'does not modify emails for users belonging to multiple accounts' do