Add resumable_job.reconnect_failure metric for failed reconnects

[1fanwang]

A ResumableJobMixin retry increments resumable_job.reconnect_attempt and then one
of reconnect_success / already_succeeded / terminal_resubmit, but only once the
prior job's status has been fetched and classified. When that status check raises
(external system unreachable, expired id, and so on) the attempt resolves to no
outcome: the failure is silent and reconnect_attempt no longer reconciles with its
outcome counters. Reconnection failure is the signal an operator most wants to
alert on, yet today it cannot be measured.

resumable_job.reconnect_failure is emitted when the reconnect decision raises, so
every reconnect_attempt resolves to exactly one outcome.

Testing

Unit: a parametrized regression test covers the status-fetch and status-classification
raise paths (fails on main, passes here); the full test_resumablejobmixin.py suite is
green (42 passed).

E2E: drove the real ResumableJobMixin against a real StatsD -> statsd-exporter ->
Prometheus -> Grafana stack. A scenario of 4 active + 3 succeeded + 2 terminal + 5
raising reconnects, plus 6 fresh submits, produced in Prometheus:

airflow_resumable_job_reconnect_attempt   14
airflow_resumable_job_reconnect_success    4
airflow_resumable_job_already_succeeded    3
airflow_resumable_job_terminal_resubmit    2
airflow_resumable_job_reconnect_failure    5
airflow_resumable_job_fresh_submit         6

reconnect_attempt (14) == reconnect_success + already_succeeded + terminal_resubmit

• reconnect_failure (4 + 3 + 2 + 5). Without this change the sum is 9 and the
  attempts do not reconcile.

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• Yes — GitHub Copilot CLI (Claude Opus 4.8)

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