feat: Linear OAuth 2.0 (#10851)

Fixes https://linear.app/chatwoot/issue/CW-3417/oauth-20-authentication
We are planning to publish the Chatwoot app in the Linear [integration
list](https://linear.app/docs/integration-directory). While we currently
use token-based authentication, Linear recommends OAuth2 authentication.
This PR implements OAuth2 support.

---------

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

spec/controllers/api/v1/accounts/integrations/linear_controller_spec.rb

@@ -12,6 +12,16 @@ RSpec.describe 'Linear Integration API', type: :request do
     allow(Integrations::Linear::ProcessorService).to receive(:new).with(account: account).and_return(processor_service)
   end
 
+  describe 'DELETE /api/v1/accounts/:account_id/integrations/linear' do
+    it 'deletes the linear integration' do
+      delete "/api/v1/accounts/#{account.id}/integrations/linear",
+             headers: agent.create_new_auth_token,
+             as: :json
+      expect(response).to have_http_status(:ok)
+      expect(account.hooks.count).to eq(0)
+    end
+  end
+
   describe 'GET /api/v1/accounts/:account_id/integrations/linear/teams' do
     context 'when it is an authenticated user' do
       context 'when data is retrieved successfully' do

spec/controllers/linear/callbacks_controller_spec.rb

@@ -0,0 +1,88 @@
+require 'rails_helper'
+
+RSpec.describe Linear::CallbacksController, type: :request do
+  let(:account) { create(:account) }
+  let(:code) { SecureRandom.hex(10) }
+  let(:state) { SecureRandom.hex(10) }
+  let(:linear_redirect_uri) { "#{ENV.fetch('FRONTEND_URL', '')}/app/accounts/#{account.id}/settings/integrations/linear" }
+
+  describe 'GET /linear/callback' do
+    let(:access_token) { SecureRandom.hex(10) }
+    let(:response_body) do
+      {
+        'access_token' => access_token,
+        'token_type' => 'Bearer',
+        'expires_in' => 7200,
+        'scope' => 'read,write'
+      }
+    end
+
+    before do
+      stub_const('ENV', ENV.to_hash.merge('FRONTEND_URL' => 'http://www.example.com'))
+
+      controller = described_class.new
+      allow(controller).to receive(:verify_linear_token).with(state).and_return(account.id)
+      allow(described_class).to receive(:new).and_return(controller)
+    end
+
+    context 'when successful' do
+      before do
+        stub_request(:post, 'https://api.linear.app/oauth/token')
+          .to_return(
+            status: 200,
+            body: response_body.to_json,
+            headers: { 'Content-Type' => 'application/json' }
+          )
+      end
+
+      it 'creates a new integration hook' do
+        expect do
+          get linear_callback_path, params: { code: code, state: state }
+        end.to change(Integrations::Hook, :count).by(1)
+
+        hook = Integrations::Hook.last
+        expect(hook.access_token).to eq(access_token)
+        expect(hook.app_id).to eq('linear')
+        expect(hook.status).to eq('enabled')
+        expect(hook.settings).to eq(
+          'token_type' => 'Bearer',
+          'expires_in' => 7200,
+          'scope' => 'read,write'
+        )
+        expect(response).to redirect_to(linear_redirect_uri)
+      end
+    end
+
+    context 'when the code is missing' do
+      before do
+        stub_request(:post, 'https://api.linear.app/oauth/token')
+          .to_return(
+            status: 200,
+            body: response_body.to_json,
+            headers: { 'Content-Type' => 'application/json' }
+          )
+      end
+
+      it 'redirects to the linear_redirect_uri' do
+        get linear_callback_path, params: { state: state }
+        expect(response).to redirect_to(linear_redirect_uri)
+      end
+    end
+
+    context 'when the token is invalid' do
+      before do
+        stub_request(:post, 'https://api.linear.app/oauth/token')
+          .to_return(
+            status: 400,
+            body: { error: 'invalid_grant' }.to_json,
+            headers: { 'Content-Type' => 'application/json' }
+          )
+      end
+
+      it 'redirects to the linear_redirect_uri' do
+        get linear_callback_path, params: { code: code, state: state }
+        expect(response).to redirect_to(linear_redirect_uri)
+      end
+    end
+  end
+end