Count post-deferral scheduled tasks toward include_deferred pool slots

[cipheraxat]

Summary

When a pool has include_deferred=True, deferred tasks correctly occupy slots, but after the trigger fires the task instance goes deferred → scheduled and previously stopped counting until it was queued/running again. That undercounts pool usage and weakens pools as a semaphore around external resources (the motivation for including deferred tasks in the first place).

This change counts post-deferral scheduled task instances — those with next_method set — toward pool occupancy when include_deferred is enabled:

• Pool.occupied_slots() / Pool.slots_stats() open calculation
• Pool.task_instance_occupies_slot() used by PoolSlotsAvailableDep so a TI that already holds a slot does not block itself

First-lifetime scheduled tasks (no next_method) are not counted, which would otherwise deadlock scheduling when open = total - scheduled.

• closes: Airflow pools do not count tasks in scheduled state #69187

Test plan

• test_open_slots_including_deferred_counts_post_deferral_scheduled
• test_open_slots_excluding_deferred_ignores_post_deferral_scheduled
• test_post_deferral_scheduled_pooled_task_pass (pool slots available dep)
• Existing pool / deferred occupancy tests still pass
• prek on changed files

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