feat: Backend - Companies API endpoint with pagination and search (#12840)

## Description

Adds API endpoint to list companies with pagination, search, and
sorting.

Fixes
https://linear.app/chatwoot/issue/CW-5930/add-backend-routes-to-get-companies-result
Parent issue:
https://linear.app/chatwoot/issue/CW-5928/add-companies-tab-to-dashboard

## Type of change

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

## How Has This Been Tested?

Added comprehensive specs to
`spec/enterprise/controllers/api/v1/accounts/companies_controller_spec.rb`:
- Pagination (25 per page, multiple pages)
- Search by name and domain (case-insensitive)
- Counter cache for contacts_count
- Account scoping
- Authorization

To reproduce:
```bash
bundle exec rspec spec/enterprise/controllers/api/v1/accounts/companies_controller_spec.rb
bundle exec rubocop enterprise/app/controllers/api/v1/accounts/companies_controller.rb
```

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] 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
- [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

---------

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
Co-authored-by: iamsivin <iamsivin@gmail.com>
Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>
Co-authored-by: Sojan Jose <sojan@pepalo.com>

app/jobs/migration/backfill_companies_contacts_count_job.rb

@@ -0,0 +1,13 @@
+class Migration::BackfillCompaniesContactsCountJob < ApplicationJob
+  queue_as :async_database_migration
+
+  def perform
+    return unless ChatwootApp.enterprise?
+
+    Company.find_in_batches(batch_size: 100) do |company_batch|
+      company_batch.each do |company|
+        Company.reset_counters(company.id, :contacts)
+      end
+    end
+  end
+end