Return each attempt's own URL from operator extra links

[1fanwang]

TL;DR — operator extra links don't work per attempt. After a retry, the link button under
attempt 1 opens attempt 2's logs, and attempt 1's logs cannot be reached from the UI at all.

Closes #71471.

It hits every bundled provider whose link carries a job id the remote service mints per
submission: EMR, Glue, Databricks, Dataproc, Livy.

Why it happens

A link is one XCom row under BaseOperatorLink.xcom_key. XComOperatorLink.get_link receives
a TaskInstanceKey and ignores its try_number, and that row only ever holds the latest
attempt anyway — the run endpoint clears every XCom for the task before each attempt.

#65661 made the extra-links endpoint resolve the requested attempt, which is what makes the
wrong answer reachable from the UI.

The fix

The worker also writes each attempt's rendered link to the task state store, which a retry
does not clear — that is what it exists for, and what the durable-execution work in #69914 and
#71211 relies on. The reader prefers that row and falls back to the XCom row, which stays in
place and is still the answer for links written before this change.

Reading goes through get_state_backend(), the same resolver the execution API uses, so a
deployment pointing [state_store] backend elsewhere keeps working.

TaskScope is keyed on (dag_id, run_id, task_id, map_index) with no attempt, so the try is
encoded in the key. The store buys immunity from the clear, not a per-attempt dimension.

Testing

Red to green on a real 3.3.0 install, store populated per attempt and XCom holding the latest.

Raw output

Before — attempt 1 resolves to attempt 2's URL:

$ AIRFLOW__DATABASE__SQL_ALCHEMY_CONN=sqlite:///af.db python c35.py
  try 1 -> https://logs/attempt-2
  try 2 -> https://logs/attempt-2

After:

$ AIRFLOW__DATABASE__SQL_ALCHEMY_CONN=sqlite:///af.db python c35.py
  try 1 -> https://logs/attempt-1
  try 2 -> https://logs/attempt-2

Tests cover both sides, run locally against a source install:

$ pytest airflow-core/tests/unit/serialization/definitions/test_operatorlink.py -q
4 passed

$ pytest task-sdk/tests/task_sdk/execution_time/test_task_runner.py -k link_per_attempt -q
1 passed

They cover the reader returning the requested attempt, falling back to XCom, preferring the
store over the XCom row that holds the latest attempt, and returning empty when nothing is
stored; plus finalize writing the attempt's link to the store.

The existing link tests still pass. Two failures in test_dag_serialization.py reproduce on
unmodified main, so they are not from this change.

Notes for review

Two alternatives are drafted if you would rather not put link data in the state store: a row
per attempt in XCom exempt from the clear (#71518), or the same with the link opting in
(#71521). I'll close both once this direction is confirmed.

One constraint worth naming: the state store landed after 3.2.2, so this cannot be backported
to the 3.2 line. #71518 can, if that matters.

No newsfragment yet — happy to add one if you want it.