Fix ExternalTaskSensor to work correctly with task groups

[dzhigimont]

Fix ExternalTaskSensor to work correctly with task groups that have mapped tasks.
Unit tests were added.
Closes: #30689

[dzhigimont]

@uranusjr Could you please review?

[potiuk]

LGTM. @uranusjr ?

[dzhigimont]

@uranusjr Ping, could you please take a look? 🙃

[uranusjr]

This looks fine. I’m wondering though, instead of changing the logic to count (mapped) task instances, would it be easier to use something like GROUP BY to count tasks? In other words, if a task is mapped, we count 1 when and only when all the mapped instances of the task are finished, and 0 otherwise. This shouldn’t affect the general behaviour of the sensor, but I think can save at least one database query (if my mental model on this is correct).

[dzhigimont]

This looks fine. I’m wondering though, instead of changing the logic to count (mapped) task instances, would it be easier to use something like GROUP BY to count tasks? In other words, if a task is mapped, we count 1 when and only when all the mapped instances of the task are finished, and 0 otherwise. This shouldn’t affect the general behaviour of the sensor, but I think can save at least one database query (if my mental model on this is correct).

@uranusjr Hi, I think it's not so easy to write that query, and it can be overcomplicated. For me, that overcomplication doesn't cost of saving one not heavy query.

[uranusjr]

OK. I guess we could always optimise things, the proposed implementation is better than things being broken.