feat: Add company auto-association for contacts (CW-5726 Part 2) (#12711)

## Description

Implements real-time company auto-association for contacts based on
email domains. This is **Part 2** of the company model production
rollout (CW-5726).

**Task:**
- When a contact is created with a business email, automatically create
and associate a company from the email domain
- When a contact is updated with an email for the first time (email was
previously nil), associate with a company
- Preserve existing company associations when email changes to avoid
user confusion
- Skip free email providers and disposable domains

**Dependencies:**
⚠️ Requires PR #12657 (Part 1: Backfill migration) to be merged first

**Linear ticket:**
[CW-5726](https://linear.app/chatwoot/issue/CW-5726/company-model-setting-it-up-on-production)

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

- Service specs: Tests business email detection, company creation,
association logic, edge cases (existing companies, free emails, nil
emails)
- Integration specs: Tests full callback flow for contact create/update
scenarios
- All tests passing: 10 examples, 0 failures
- RuboCop: 0 offenses

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules (PR #12657 pending)

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

spec/enterprise/jobs/migration/company_account_batch_job_spec.rb

@@ -19,6 +19,12 @@ RSpec.describe Migration::CompanyAccountBatchJob, type: :job do
       let!(:contact) { create(:contact, account: account, email: 'user@acme.com') }
 
       it 'creates a company and associates the contact' do
+        # Clean up companies created by Part 2's callback
+        Company.delete_all
+        # rubocop:disable Rails/SkipsModelValidations
+        contact.update_column(:company_id, nil)
+        # rubocop:enable Rails/SkipsModelValidations
+
         expect do
           described_class.perform_now(account)
         end.to change(Company, :count).by(1)
@@ -71,6 +77,13 @@ RSpec.describe Migration::CompanyAccountBatchJob, type: :job do
       let!(:contact2) { create(:contact, account: account, email: 'user2@acme.com') }
 
       it 'creates only one company for the domain' do
+        # Clean up companies created by Part 2's callback
+        Company.delete_all
+        # rubocop:disable Rails/SkipsModelValidations
+        contact1.update_column(:company_id, nil)
+        contact2.update_column(:company_id, nil)
+        # rubocop:enable Rails/SkipsModelValidations
+
         expect do
           described_class.perform_now(account)
         end.to change(Company, :count).by(1)