Add get letter contact by id endpoint

Added the GET /<uuid:service_id>/letter-contact/<uuid:letter_contact_id>
endpoint for returning a letter contact for a service with the letter contact id

Added the DAO for returning a letter contact by id

tests/app/dao/test_service_letter_contact_dao.py

@@ -1,9 +1,12 @@
+import uuid
 import pytest
+from sqlalchemy.exc import SQLAlchemyError
 
 from app.dao.service_letter_contact_dao import (
     add_letter_contact_for_service,
     create_or_update_letter_contact,
     dao_get_letter_contacts_by_service_id,
+    dao_get_letter_contact_by_id,
     update_letter_contact
 )
 from app.errors import InvalidRequest
@@ -202,3 +205,20 @@ def test_update_letter_contact_unset_default_for_only_letter_contact_raises_exce
             contact_block='Warwick, W14 TSR',
             is_default=False
         )
+
+
+def test_dao_get_letter_contact_by_id(sample_service):
+    letter_contact = create_letter_contact(service=sample_service, contact_block='Aberdeen, AB12 23X')
+    result = dao_get_letter_contact_by_id(service_id=sample_service.id, letter_contact_id=letter_contact.id)
+    assert result == letter_contact
+
+
+def test_dao_get_letter_contact_by_id_raises_sqlalchemy_error_when_letter_contact_does_not_exist(sample_service):
+    with pytest.raises(SQLAlchemyError):
+        dao_get_letter_contact_by_id(service_id=sample_service.id, letter_contact_id=uuid.uuid4())
+
+
+def test_dao_get_letter_contact_by_id_raises_sqlalchemy_error_when_service_does_not_exist(sample_service):
+    letter_contact = create_letter_contact(service=sample_service, contact_block='Some address')
+    with pytest.raises(SQLAlchemyError):
+        dao_get_letter_contact_by_id(service_id=uuid.uuid4(), letter_contact_id=letter_contact.id)