Ensure deterministic ordering in DB queries to prevent flaky tests

[MilanTyagi2004]

Title: [#13415] Non-deterministic ordering in DB queries causes inconsistent API behavior and flaky tests

Fixes #13415

Outline of Solution

• Enforce deterministic ordering on collection queries in the SQL layer
  (UsersDb, AccountRequestsDb, NotificationsDb, FeedbackSessionsDb, AccountsDb)
  using explicit ORDER BY.

• Enforce deterministic ordering on Objectify/Datastore queries
  (StudentsDb, InstructorsDb, FeedbackSessionsDb, AccountsDb)
  via .order(...).

• Remove redundant in-memory sorting in UsersLogic now that DB queries return
  ordered results.

[github-actions]

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