Add explicit session parameter in PoolSlotsAvailableDep

[avkirilishin]

An open_slots function accepts a session parameter, and it should not commit the transaction itself. The absence of a parameter causes an implicit commit and may lead to undesirable consequences. For example, a task may become orphaned if restarting the scheduler and task creation happens around the same time.

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[boring-cyborg]

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[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[potiuk]

Oh. Nice!

[avkirilishin]

I reduced the number of queries in the test. Why are two excluded instead of one?
First - implicit COMMIT (expected).
Second - SELECT in new session (SELECT ... FROM task_instance JOIN dag_run ...).

[uranusjr]

If you're just curious, try changing the expected query count in the test against main and see what assert_queries_count reports 🙂 But less query is always good so that shouldn't affect this PR.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!