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/models/contact_company_association_spec.rb

@@ -0,0 +1,61 @@
+require 'rails_helper'
+
+RSpec.describe Contact, type: :model do
+  describe 'company auto-association' do
+    let(:account) { create(:account) }
+
+    context 'when creating a new contact with business email' do
+      it 'automatically creates and associates a company' do
+        expect do
+          create(:contact, email: 'john@acme.com', account: account)
+        end.to change(Company, :count).by(1)
+        contact = described_class.last
+        expect(contact.company).to be_present
+        expect(contact.company.domain).to eq('acme.com')
+      end
+
+      it 'does not create company for free email providers' do
+        expect do
+          create(:contact, email: 'john@gmail.com', account: account)
+        end.not_to change(Company, :count)
+      end
+    end
+
+    context 'when updating a contact to add email for first time' do
+      it 'creates and associates company' do
+        contact = create(:contact, email: nil, account: account)
+        expect do
+          contact.update(email: 'john@acme.com')
+        end.to change(Company, :count).by(1)
+        contact.reload
+        expect(contact.company.domain).to eq('acme.com')
+      end
+    end
+
+    context 'when updating a contact that already has a company' do
+      it 'does not change company when email changes' do
+        existing_company = create(:company, domain: 'oldcompany.com', account: account)
+        contact = create(:contact, email: 'john@oldcompany.com', company: existing_company, account: account)
+
+        expect do
+          contact.update(email: 'john@new_company.com')
+        end.not_to change(Company, :count)
+        contact.reload
+        expect(contact.company).to eq(existing_company)
+      end
+    end
+
+    context 'when multiple contacts share the same domain' do
+      it 'associates all contacts with the same company' do
+        contacts = ['john@acme.com', 'jane@acme.com', 'bob@acme.com']
+        contacts.each do |contact|
+          create(:contact, email: contact, account: account)
+        end
+
+        expect(Company.where(domain: 'acme.com', account: account).count).to eq(1)
+        company = Company.find_by(domain: 'acme.com', account: account)
+        expect(company.contacts.count).to eq(contacts.length)
+      end
+    end
+  end
+end