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>

db/migrate/20251111094402_add_contacts_count_to_companies.rb

@@ -0,0 +1,5 @@
+class AddContactsCountToCompanies < ActiveRecord::Migration[7.1]
+  def change
+    add_column :companies, :contacts_count, :integer, default: 0, null: false
+  end
+end

db/schema.rb

@@ -577,6 +577,7 @@ ActiveRecord::Schema[7.1].define(version: 2025_11_14_173609) do
     t.bigint "account_id", null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.integer "contacts_count"
     t.index ["account_id", "domain"], name: "index_companies_on_account_and_domain", unique: true, where: "(domain IS NOT NULL)"
     t.index ["account_id"], name: "index_companies_on_account_id"
     t.index ["name", "account_id"], name: "index_companies_on_name_and_account_id"