Improve LocalExecutor bookkeeping: correctly add tasks to the running list

[ashb]

Since #73916 landed, BaseExecutor now keeps a UUID-to-coordinates map for each
attempt and drops an entry once the key is no longer queued, running or in the
event buffer. The issue was that LocalExecutor never added dispatched work to
running, so the entry was dropped while the task was still executing. The
scheduler then received the final event with no coordinates ("Received executor
event with state success for task instance (coordinates=None)").

This PR fixes that, and addresses a few gotchas that could, in edge cases
around callbacks or post-task finalization, lead to a forever-dead/locked
worker slot.

Holding the key in running until the worker finishes needs LocalExecutor
to know when that is, and workers share one activity queue so the parent
cannot tell which worker took which workload. Results now carry the
worker's pid and START tells the parent which pid owns the key. That lets
us:

• release the slot only on a terminal result from the owning worker,
  ignoring results from unknown pids or pids that don't own the key
• fail the workload when its worker dies, instead of leaving it in
  running until the scheduler's heartbeat timeout

A resumed task reuses its key, so a per-key dispatch count stops the old
run's terminal result from releasing the new run's slot.

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

---

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}

[ashb]

Addressing the issue raised by Shahar in #73916 (comment)

[fat-catTW]

Awesome!