Added endpoint to get the letter preview from the template preview app

Is will allow the admin application to call into the api instead of
making the call itself. This will allow the api to make decision for
precompiled pdf without having to update the admin app.

- Added new endpoint
- Added tests for the endpoint

tests/app/template/test_rest.py

@@ -6,6 +6,7 @@ from datetime import datetime, timedelta
 import pytest
 from freezegun import freeze_time
 
+from app.errors import InvalidRequest
 from app.models import Template, SMS_TYPE, EMAIL_TYPE, LETTER_TYPE, TemplateHistory
 from app.dao.templates_dao import dao_get_template_by_id, dao_redact_template
 
@@ -14,9 +15,9 @@ from tests.app.conftest import (
     sample_template as create_sample_template,
     sample_template_without_email_permission,
     sample_template_without_letter_permission,
-    sample_template_without_sms_permission,
-)
+    sample_template_without_sms_permission)
 from tests.app.db import create_service, create_letter_contact, create_template
+from tests.conftest import set_config_values
 
 
 @pytest.mark.parametrize('template_type, subject', [
@@ -794,3 +795,87 @@ def test_update_redact_template_400s_if_no_created_by(admin_request, sample_temp
 
     assert sample_template.redact_personalisation is False
     assert sample_template.template_redacted.updated_at == original_updated_time
+
+
+def test_preview_letter_template_by_id_invalid_file_type(
+        sample_service,
+        sample_letter_template,
+        sample_letter_notification):
+
+    with pytest.raises(InvalidRequest) as exc:
+        from app.template.rest import preview_letter_template_by_notification_id
+        preview_letter_template_by_notification_id(
+            sample_service.id,
+            sample_letter_template.id,
+            sample_letter_notification.id,
+            'doc')
+    assert 'file_type must be pdf or png' in str(exc.value)
+
+
+def test_preview_letter_template_by_id_valid_file_type(
+        notify_api,
+        client,
+        sample_service,
+        sample_letter_template,
+        sample_letter_notification):
+
+    with set_config_values(notify_api, {
+        'TEMPLATE_PREVIEW_API_HOST': 'http://localhost/notifications-template-preview',
+        'TEMPLATE_PREVIEW_API_KEY': 'test-key'
+    }):
+        import requests_mock
+        with requests_mock.Mocker() as request_mock:
+            content = b'\x00\x01'
+
+            request_mock.post(
+                'http://localhost/notifications-template-preview/preview.pdf',
+                content=content,
+                headers={'X-pdf-page-count': '1'},
+                status_code=200
+            )
+
+            from app.template.rest import preview_letter_template_by_notification_id
+            content, status_code, items = preview_letter_template_by_notification_id(
+                sample_service.id,
+                sample_letter_template.id,
+                sample_letter_notification.id,
+                'pdf'
+            )
+
+            assert status_code == 200
+            assert content == content
+            assert list(items)[0][0] == 'X-pdf-page-count'
+            assert list(items)[0][1] == '1'
+
+
+def test_preview_letter_template_by_id_template_preview_404(
+        notify_api,
+        client,
+        sample_service,
+        sample_letter_template,
+        sample_letter_notification):
+
+    with set_config_values(notify_api, {
+        'TEMPLATE_PREVIEW_API_HOST': 'http://localhost/notifications-template-preview',
+        'TEMPLATE_PREVIEW_API_KEY': 'test-key'
+    }):
+        import requests_mock
+        with requests_mock.Mocker() as request_mock:
+            content = b'\x00\x01'
+
+            request_mock.post(
+                'http://localhost/notifications-template-preview/preview.pdf',
+                content=content,
+                headers={'X-pdf-page-count': '1'},
+                status_code=404
+            )
+
+            from app.template.rest import preview_letter_template_by_notification_id
+            content, status_code, items = preview_letter_template_by_notification_id(
+                sample_service.id,
+                sample_letter_template.id,
+                sample_letter_notification.id,
+                'pdf'
+            )
+
+            assert status_code == 404