fix: Only merge activities on computed parent station informed entities, not original child stop informed entities

[jzimbel-mbta]

Summary of changes

Asana Ticket: 🍎Alert informed entities should have activities as specified in the API

Overall summary of the change: the activities-merging logic is now applied exclusively to computed informed entities for parent stations, ensuring that the original informed entities are preserved.

I added a bunch of tests for this complex logic to be sure I didn't cause any other unintended changes in behavior.

Results of running the new tests against current main branch, without this change:

$ mix test apps/state/test/state/alert/hooks_test.exs
==> state
Running ExUnit with seed: 330364, max_cases: 22

.

  1) test pre_insert_hook/1 handles specific case from bugfix ticket (State.Alert.HooksTest)
     apps/state/test/state/alert/hooks_test.exs:353
     match (=) failed
     The following variables were pinned:
       informed_entities = [%{stop: "WR-0045-S", route: "CR-Haverhill", direction_id: 0, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]}, %{stop: "WR-0045-S", route: "CR-Haverhill", direction_id: 1, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]}, %{stop: "BNT-0000", route: "CR-Haverhill", direction_id: 0, route_type: 2, activities: ["BOARD", "RIDE"]}, %{stop: "WR-0053-S", route: "CR-Haverhill", direction_id: 1, route_type: 2, activities: ["BOARD", "RIDE"]}, %{stop: "WR-0053-S", route: "CR-Haverhill", direction_id: 0, route_type: 2, activities: ["EXIT", "RIDE"]}, %{stop: "BNT-0000", route: "CR-Haverhill", direction_id: 1, route_type: 2, activities: ["EXIT", "RIDE"]}]
       expected_new_informed_entities = [%{stop: "place-mlmnl", route: "CR-Haverhill", direction_id: 0, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]}, %{stop: "place-north", route: "CR-Haverhill", direction_id: 0, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]}, %{stop: "place-ogmnl", route: "CR-Haverhill", direction_id: 0, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]}, %{stop: "place-mlmnl", route: "CR-Haverhill", direction_id: 1, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]}, %{stop: "place-north", route: "CR-Haverhill", direction_id: 1, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]}, %{stop: "place-ogmnl", route: "CR-Haverhill", direction_id: 1, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]}]
     code:  assert %{preserved: ^informed_entities, added: ^expected_new_informed_entities} =
              get_computed_informed_entities(alert)
     left:  %{added: ^expected_new_informed_entities, preserved: ^informed_entities}
     right: %{
              added: [
                %{stop: "BNT-0000", route: "CR-Haverhill", direction_id: 0, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]},
                %{stop: "WR-0053-S", route: "CR-Haverhill", direction_id: 0, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]},
                %{activities: ["BOARD", "EXIT", "RIDE"], direction_id: 0, route: "CR-Haverhill", route_type: 2, stop: "place-mlmnl"},
                %{activities: ["BOARD", "EXIT", "RIDE"], direction_id: 0, route: "CR-Haverhill", route_type: 2, stop: "place-north"},
                %{activities: ["BOARD", "EXIT", "RIDE"], direction_id: 0, route: "CR-Haverhill", route_type: 2, stop: "place-ogmnl"},
                %{activities: ["BOARD", "EXIT", "RIDE"], direction_id: 1, route: "CR-Haverhill", route_type: 2, stop: "BNT-0000"},
                %{stop: "WR-0053-S", route: "CR-Haverhill", direction_id: 1, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]},
                %{stop: "place-mlmnl", route: "CR-Haverhill", direction_id: 1, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]},
                %{stop: "place-north", route: "CR-Haverhill", direction_id: 1, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]},
                %{stop: "place-ogmnl", route: "CR-Haverhill", direction_id: 1, route_type: 2, activities: ["BOARD", "EXIT", "RIDE"]}
              ],
              preserved: [
                %{activities: ["BOARD", "EXIT", "RIDE"], direction_id: 0, route: "CR-Haverhill", route_type: 2, stop: "WR-0045-S"},
                %{activities: ["BOARD", "EXIT", "RIDE"], direction_id: 1, route: "CR-Haverhill", route_type: 2, stop: "WR-0045-S"}
              ],
              removed: [%{stop: "BNT-0000", route: "CR-Haverhill", direction_id: 0, route_type: 2, activities: ["BOARD", "RIDE"]}, %{stop: "WR-0053-S", route: "CR-Haverhill", direction_id: 1, route_type: 2, activities: ["BOARD", "RIDE"]}, %{stop: "WR-0053-S", route: "CR-Haverhill", direction_id: 0, route_type: 2, activities: ["EXIT", "RIDE"]}, %{stop: "BNT-0000", route: "CR-Haverhill", direction_id: 1, route_type: 2, activities: ["EXIT", "RIDE"]}]
            }
     stacktrace:
       test/state/alert/hooks_test.exs:413: (test)

...
Finished in 0.1 seconds (0.00s async, 0.1s sync)
5 tests, 1 failure