Make dependency failure handling more consistent

This moves code around a little bit to make error handling in the
dependency failure case more consistent with other failures. This
unlocks potential for refactoring of the error handling code.

Rought principles for error handling that are true for other errors
and that this PR makes true for dependency failures:

* _launch_if_ready_async creates a Future for the task, and does not
  modify the TaskRecord state or send monitoring information

* handle_exec_update decides if a task has failed by whether the
  Future contains an exception or not. It updates the TaskRecord
  state and sends monitoring information.

Removing one call of _send_task_log_info (in _launch_if_ready_async)
and instead relying on the existing call in handle_exec_update
removes a duplicate dep_fail entry in the monitoring database, that
was previously unnoticed.

$ pytest parsl/tests/test_python_apps/test_depfail_propagation.py --config parsl/tests/configs/htex_local_alternate.py

Before this PR:

sqlite> select * from status where task_status_name = 'dep_fail';
1|dep_fail|2025-10-16 09:29:20.171145|1d65bd94-8639-4ebf-90f9-2b61cca36763|0
1|dep_fail|2025-10-16 09:29:20.171472|1d65bd94-8639-4ebf-90f9-2b61cca36763|0
3|dep_fail|2025-10-16 09:29:20.255851|1d65bd94-8639-4ebf-90f9-2b61cca36763|0
3|dep_fail|2025-10-16 09:29:20.256067|1d65bd94-8639-4ebf-90f9-2b61cca36763|0
4|dep_fail|2025-10-16 09:29:20.256353|1d65bd94-8639-4ebf-90f9-2b61cca36763|0
4|dep_fail|2025-10-16 09:29:20.256452|1d65bd94-8639-4ebf-90f9-2b61cca36763|0
5|dep_fail|2025-10-16 09:29:20.256732|1d65bd94-8639-4ebf-90f9-2b61cca36763|0
5|dep_fail|2025-10-16 09:29:20.256856|1d65bd94-8639-4ebf-90f9-2b61cca36763|0
7|dep_fail|2025-10-16 09:29:20.335936|1d65bd94-8639-4ebf-90f9-2b61cca36763|0
7|dep_fail|2025-10-16 09:29:20.336110|1d65bd94-8639-4ebf-90f9-2b61cca36763|0
8|dep_fail|2025-10-16 09:29:20.336398|1d65bd94-8639-4ebf-90f9-2b61cca36763|0
8|dep_fail|2025-10-16 09:29:20.336522|1d65bd94-8639-4ebf-90f9-2b61cca36763|0

After this PR:

sqlite> select * from status where task_status_name = 'dep_fail';
1|dep_fail|2025-10-16 09:31:35.782225|be2cd195-b13b-4b03-ac2f-95b73cc9a200|0
3|dep_fail|2025-10-16 09:31:35.865129|be2cd195-b13b-4b03-ac2f-95b73cc9a200|0
4|dep_fail|2025-10-16 09:31:35.865509|be2cd195-b13b-4b03-ac2f-95b73cc9a200|0
5|dep_fail|2025-10-16 09:31:35.865873|be2cd195-b13b-4b03-ac2f-95b73cc9a200|0
7|dep_fail|2025-10-16 09:31:35.945858|be2cd195-b13b-4b03-ac2f-95b73cc9a200|0
8|dep_fail|2025-10-16 09:31:35.946355|be2cd195-b13b-4b03-ac2f-95b73cc9a200|0

Author: benclifford