feat(metrics): emit dagrun.deadlocked counter on task-deadlock detection

[1fanwang]

Fix for the missing metric called out in #66818. On our LinkedIn DI cluster we have no signal today when DagRun.update_state detects an all-tasks-deadlocked Dag run — the case is logged but not metered, so we don't alert until a user reports it. This PR emits a dagrun.deadlocked counter on the existing detection path; no behaviour change otherwise.

Problem

DagRun.update_state() already detects the task-deadlock case — when every unfinished task is unrunnable — logs an error, and notifies the state-changed listeners with msg="all_tasks_deadlocked". It does not emit a Stats counter, so operators who want to alert on deadlock-induced failures end up grepping scheduler logs or scraping state-change notifications. There's no first-class signal alongside zombies.zombie_unfinished_run_failure_count or the executor-event failure counters.

Fix

Add stats.incr("dagrun.deadlocked", tags={"dag_id": self.dag_id, "run_type": self.run_type}) at the existing log + notify call site in DagRun.update_state, and register the new counter in the observability metrics template.

Tests

Added test_dagrun_deadlock_emits_stats_counter in airflow-core/tests/unit/models/test_dagrun.py. Mirrors the existing test_dagrun_deadlock fixture (invalid trigger_rule to force the deadlock branch), mocks stats.incr, and asserts the call with the expected name and tags.

Reproducer

The new test airflow-core/tests/unit/models/test_dagrun.py::TestDagRun::test_dagrun_deadlock_emits_stats_counter builds a two-task DagRun whose downstream task has an invalid trigger_rule, drives update_state into the deadlock branch, and asserts stats.incr was called with "dagrun.deadlocked" plus the dag_id and run_type tags.

Without the dagrun.py change (reverted to upstream/main), the test fails on the mock assertion:

FAILED airflow-core/tests/unit/models/test_dagrun.py::TestDagRun::test_dagrun_deadlock_emits_stats_counter
AssertionError: incr('dagrun.deadlocked', tags={'dag_id': 'test_dag', 'run_type': <DagRunType.MANUAL: 'manual'>}) call not found

With the stats.incr call restored, the test passes:

PASSED airflow-core/tests/unit/models/test_dagrun.py::TestDagRun::test_dagrun_deadlock_emits_stats_counter
======================== 1 passed, 1 warning in 15.88s =========================

The deadlock branch is reached either way — Task deadlock (no runnable tasks); marking run ... failed appears in both runs — confirming the FAILED→PASSED transition is driven by the new stats.incr line, not by a difference in which code path executes.

Closes #66818