fix: choose correct Salary Structure Assignment when getting data for formula eval

[sagarvora]

Resolves #24601
Resolves #21307

Changes Made

• Improved validation for Payroll Period to see if employee hasn't joined or has already left
• Choosing correct salary structure with following additional filters:
  • Salary Structure Assignment's From Date should be less than or equal to (Payroll Period Start Date or Employee Joining Date whichever is later)
  • Salary Structure Assignment should be submitted
• Gets the Salary Structure Assignment with From Date closest to Payroll Start Date / Employee Joining Date instead of choosing the one which was last modified (order_by)
• Error message if Salary Structure Assignment with above filters not found

[sagarvora]

Changes have been tested locally.

[sagarvora]

Breaking tests are unrelated. Directly linked tests have been fixed.

[sagarvora]

• This code has been added because after newly introduced validation, a Salary Structure Assignment must be applicable on or before the Salary Slip date.
• This code assumes that employee has joined before Payroll Period. Alternatively, from_date can be manually passed to make_salary_structure in these cases.

[ruchamahabal]

LGTM

[ruchamahabal]

@sagarvora Can you also raise a fix for version-13-hotfix branch?