[v3-3-test] Make deadline reads and serialization robust to dynamic/malformed intervals (#68919)

[github-actions]

• Make deadline reads and serialization robust to dynamic/malformed intervals

Hardens the read and (de)serialization paths for deadline alerts so dynamic
(VariableInterval) and malformed stored data no longer break the UI/API.

• UI deadline-alert response: DeadlineAlert.interval is a JSON column holding
  the Airflow-serialized interval, not a plain number. Coerce it to seconds for a
  fixed timedelta and to None for a dynamic VariableInterval (resolved
  later by the scheduler), instead of letting Pydantic 500 on the dict. The
  interval field becomes float | None.
• Drop interval from the sortable columns of the deadline-alerts endpoint:
  ordering by a JSON column sorts by structure/text, not duration, so the result
  was arbitrary and misleading.
• Deserialization: route by the encoder-stamped __class_path ahead of the
  reference_type name (a custom reference may share a class name with a
  builtin), and raise a clear error for a reference with no importable
  __class_path instead of an opaque KeyError.
• Deadline.__repr__ / DeadlineAlert.__repr__ no longer raise: guard the
  dagrun relationship (the FK can be set while the relationship is None after a
  cascade delete) and handle the dict-shaped JSON interval. A __repr__ must
  never raise.
• prune_deadlines explicitly excludes deadlines already marked missed so a
  missed deadline (whose callback is owned by the scheduler/triggerer) and its
  queued callback are never cascade-deleted.

Generated-by: Claude Code (Opus via Claude Code) on behalf of Sean Ghaeli

• Remove redundant comments in deadline sort/deserialize

The sort-key comment restated a constraint already covered by
test_order_by_interval_is_rejected; the __class_path comment restated the
ValueError message right below it.

Generated-by: Claude Code (Opus)

• Trim PR to the UI deadlineAlerts 500 fix only

Per review: keep only the interval coercion, order_by removal, and their
tests; move repr guards, decoder routing, and prune guard to a follow-up.

---

(cherry picked from commit f840a82)

Co-authored-by: Sean Ghaeli 58916776+seanghaeli@users.noreply.github.com
Co-authored-by: Sean Ghaeli ghaeli@amazon.com