fix: add duration_certainty to AlertView

[boringcactus]

Summary of changes

Asana Ticket: 🍎 Surface 'duration_certainty' in v3 API

https://api-v3.mbta.com/alerts?fields[alert]=duration_certainty contains no actual duration_certainty values, because #828 was missing this piece. I've checked that this causes duration_certainty values to be returned.

[github-actions]

Coverage of commit 88d6e97

Summary coverage rate:
  lines......: 89.1% (4223 of 4740 lines)
  functions..: 71.0% (2261 of 3186 functions)
  branches...: no data found

Files changed coverage rate:
                                                                        |Lines       |Functions  |Branches    
  Filename                                                              |Rate     Num|Rate    Num|Rate     Num
  ============================================================================================================
  apps/api_web/lib/api_web/views/alert_view.ex                          | 100%     23|80.6%    62|    -      0

Download coverage report

[github-actions]

Coverage of commit 9808fd8

Summary coverage rate:
  lines......: 89.1% (4224 of 4740 lines)
  functions..: 71.0% (2261 of 3186 functions)
  branches...: no data found

Files changed coverage rate:
                                                                        |Lines       |Functions  |Branches    
  Filename                                                              |Rate     Num|Rate    Num|Rate     Num
  ============================================================================================================
  apps/api_web/lib/api_web/views/alert_view.ex                          | 100%     23|80.6%    62|    -      0

Download coverage report