refactor: fetch all employees that may have shift type as active shift (backport #2683)

[mergify]

Problem

Some employees would miss out while marking attendance for a shift due to filters set to exclude employees that have any other active shift assignment.
#2602 was pushed to rectify this, but it would exclude employees with the same active and default shift (this is a whole other can of worms) entirely.

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Solution

Refactored to fetch all such employees who either have shift in question assigned that hasn't ended or have this shift as default shift. This may fetch some redundant employees who have another shift assigned that may have started or ended before or after the attendance processing date.
But this is done to avoid missing any employee who may have this shift as active shift.
Had added a test case with #2602.
Improved test scenario for

hrms/hrms/hr/doctype/shift_type/test_shift_type.py

Line 343 in b6f3100

def test_mark_absent_for_dates_with_no_attendance_for_midnight_shift(self):

Caveats

Could affect performance but better than having incorrect attendance records?

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This is an automatic backport of pull request #2683 done by Mergify.

[frappe-pr-bot]

🎉 This PR is included in version 15.38.1 🎉

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