Operator extra links per attempt (draft A): a row per attempt in XCom

[1fanwang]

TL;DR — operator extra links don't work per attempt (#71471). After a retry the button
under attempt 1 opens attempt 2's logs. It hits every bundled provider linking to per-attempt
logs: EMR, Glue, Databricks, Dataproc, Livy.

Draft A of three for the same bug — pick a shape and I'll finish it and close the others.
Recommended: C (#71522). See the issue for why; A is the fallback if this needs to ship on the 3.2 line.

	Approach	Trade
A	this PR — a row per attempt in XCom, exempt from the clear	smallest; core owns a key convention for every link
B	#71521 — the link opts in	no extra rows when unneeded; changes the serialized DAG format
C	#71522 — the task state store	nothing about the clear moves; 3.3+ only

How

The worker already caches each rendered link as an XCom row under the link's xcom_key. It
now also writes one under a key carrying the attempt, the reader prefers that row and falls
back to the bare key, and those rows survive the clear a retry issues.

The prefix is owned by Airflow rather than derived from xcom_key, because a link may
override that with any name — Databricks uses databricks_job_run_link, BaseGoogleLink
proxies to self.key.

Testing

Red to green on a real metastore, rows present for both attempts.

Raw output

Before, on 3.2.2 — attempt 1 resolves to attempt 2's URL:

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

After:

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

Purge predicate, including keys that would be spared if the _ were left as a LIKE wildcard:

spared : ['_link_attempt_1__link_Foo', '_link_attempt_2_databricks_job_run_link']
cleared: ['_link_Foo', 'databricks_job_run_link', 'return_value',
          'Xlink_attemptX1_y', 'my_link_attempt_1_thing']

Tests cover the reader preferring the requested attempt, the fallback, and the run endpoint
keeping those rows out of xcom_keys_to_clear. The full suite runs in CI.

[1fanwang]

Going with #71522 instead: the task state store is where cross-retry state already lives, so the XCom clear stays absolute rather than gaining an exemption. Closing this one.