Look in all parts of a letter template to find placeholders

Text messages have placeholders in their body.

Emails have them in their subject line too.

Letters have them in their body, subject line and contact block.

We were only looking in the the body and subject when processing a job,
therefore the thing assembling the letter was not looking in all the
CSV columns it needed to, because it hadn’t been told about any
placeholders in the contact block.

Fixing this means always making sure we use the correct `Template`
instance for the type of template we’re dealing with. Which we were
already doing in a different part of the codebase. So it makes sense to
reuse that.

Turns out we fixed the same bug for email subjects over 3 years ago:
3ed97151ee

tests/app/celery/test_tasks.py

@@ -9,7 +9,11 @@ from freezegun import freeze_time
 from requests import RequestException
 from sqlalchemy.exc import SQLAlchemyError
 from celery.exceptions import Retry
-from notifications_utils.template import SMSMessageTemplate, WithSubjectTemplate
+from notifications_utils.template import (
+    LetterPrintTemplate,
+    PlainTextEmailTemplate,
+    SMSMessageTemplate,
+)
 from notifications_utils.columns import Row
 
 from app import (
@@ -26,11 +30,12 @@ from app.celery.tasks import (
     save_letter,
     process_incomplete_job,
     process_incomplete_jobs,
-    get_template_class,
     s3,
     send_inbound_sms_to_service,
     process_returned_letters_list,
-    save_api_email)
+    save_api_email,
+    get_recipient_csv_and_template_and_sender_id,
+)
 from app.config import QueueNames
 from app.dao import jobs_dao, service_email_reply_to_dao, service_sms_sender_dao
 from app.models import (
@@ -1298,13 +1303,81 @@ def test_should_cancel_job_if_service_is_inactive(sample_service,
     tasks.process_row.assert_not_called()
 
 
-@pytest.mark.parametrize('template_type, expected_class', [
-    (SMS_TYPE, SMSMessageTemplate),
-    (EMAIL_TYPE, WithSubjectTemplate),
-    (LETTER_TYPE, WithSubjectTemplate),
-])
-def test_get_template_class(template_type, expected_class):
-    assert get_template_class(template_type) == expected_class
+def test_get_email_template_instance(mocker, sample_email_template, sample_job):
+    mocker.patch(
+        'app.celery.tasks.s3.get_job_and_metadata_from_s3',
+        return_value=('', {}),
+    )
+    sample_job.template_id = sample_email_template.id
+    (
+        recipient_csv,
+        template,
+        _sender_id,
+    ) = get_recipient_csv_and_template_and_sender_id(sample_job)
+
+    assert isinstance(template, PlainTextEmailTemplate)
+    assert recipient_csv.placeholders == [
+        'email address'
+    ]
+
+
+def test_get_sms_template_instance(mocker, sample_template, sample_job):
+    mocker.patch(
+        'app.celery.tasks.s3.get_job_and_metadata_from_s3',
+        return_value=('', {}),
+    )
+    sample_job.template = sample_template
+    (
+        recipient_csv,
+        template,
+        _sender_id,
+    ) = get_recipient_csv_and_template_and_sender_id(sample_job)
+
+    assert isinstance(template, SMSMessageTemplate)
+    assert recipient_csv.placeholders == [
+        'phone number'
+    ]
+
+
+def test_get_letter_template_instance(mocker, sample_job):
+    mocker.patch(
+        'app.celery.tasks.s3.get_job_and_metadata_from_s3',
+        return_value=('', {}),
+    )
+    sample_contact_block = create_letter_contact(
+        service=sample_job.service,
+        contact_block='((reference number))'
+    )
+    sample_template = create_template(
+        service=sample_job.service,
+        template_type=LETTER_TYPE,
+        reply_to=sample_contact_block.id,
+    )
+    sample_job.template_id = sample_template.id
+
+    (
+        recipient_csv,
+        template,
+        _sender_id,
+    ) = get_recipient_csv_and_template_and_sender_id(sample_job)
+
+    assert isinstance(template, LetterPrintTemplate)
+    assert template.contact_block == (
+        '((reference number))'
+    )
+    assert template.placeholders == {
+        'reference number'
+    }
+    assert recipient_csv.placeholders == [
+        'reference number',
+        'address line 1',
+        'address line 2',
+        'address line 3',
+        'address line 4',
+        'address line 5',
+        'address line 6',
+        'postcode',
+    ]
 
 
 def test_send_inbound_sms_to_service_post_https_request_to_service(notify_api, sample_service):