Ensure correct object renders V2 template preview

The `body` field of the ‘preview a template’ endpoint should contain the
content of the template with the placeholders replaced.

It should not be rendered into a plain text email, which does stuff like
normalising newlines (so `\r\n` becomes `\n`). However the object that
the `create_post_template_preview_response` function receives is an
instance of `PlainTextEmailTemplate`. So we can’t just call `str()` on
it. Instead we need to call the `__str__` method of
`WithSubjectTemplate` which gives us the result we want.

We were already doing this the right way in the V1 endpoint[1] but
forgot to do it here too.

1. 0108749daa/app/template/rest.py (L181-L184)

app/v2/template/template_schemas.py

@@ -1,3 +1,5 @@
+from notifications_utils.template import WithSubjectTemplate
+
 from app.models import SMS_TYPE, TEMPLATE_TYPES
 from app.schema_validation.definitions import uuid, personalisation
 from app.utils import get_html_email_body_from_template
@@ -75,13 +77,18 @@ post_template_preview_response = {
 
 
 def create_post_template_preview_response(template, template_object):
-    subject = template_object.subject if template.template_type != SMS_TYPE else None
+    if template.template_type == SMS_TYPE:
+        subject = None
+        body = str(template_object)
+    else:
+        subject = template_object.subject
+        body = WithSubjectTemplate.__str__(template_object)
 
     return {
         "id": template.id,
         "type": template.template_type,
         "version": template.version,
-        "body": str(template_object),
+        "body": body,
         "html": get_html_email_body_from_template(template_object),
         "subject": subject,
         "postage": template.postage

tests/app/v2/template/test_post_template.py

@@ -2,7 +2,7 @@ import pytest
 
 from flask import json
 
-from app.models import EMAIL_TYPE, SMS_TYPE, TEMPLATE_TYPES
+from app.models import EMAIL_TYPE, LETTER_TYPE, SMS_TYPE, TEMPLATE_TYPES
 from tests import create_authorization_header
 from tests.app.db import create_template
 
@@ -108,6 +108,41 @@ def test_valid_post_template_returns_200(
     assert expected_content in resp_json['body']
 
 
+@pytest.mark.parametrize("template_type", (EMAIL_TYPE, LETTER_TYPE))
+def test_email_and_letter_templates_not_rendered_into_content(
+    client,
+    sample_service,
+    template_type,
+):
+    template = create_template(
+        sample_service,
+        template_type=template_type,
+        subject='Test',
+        content=(
+            'Hello\n'
+            '\r\n'
+            '\r\n'
+            '\n'
+            '# This is a heading\n'
+            '\n'
+            'Paragraph'
+        ),
+    )
+
+    auth_header = create_authorization_header(service_id=sample_service.id)
+
+    response = client.post(
+        path='/v2/template/{}/preview'.format(template.id),
+        data=json.dumps(None),
+        headers=[('Content-Type', 'application/json'), auth_header])
+
+    assert response.status_code == 200
+
+    resp_json = json.loads(response.get_data(as_text=True))
+
+    assert resp_json['body'] == template.content
+
+
 @pytest.mark.parametrize("tmp_type", TEMPLATE_TYPES)
 def test_invalid_post_template_returns_400(client, sample_service, tmp_type):
     template = create_template(