fix: update system date format for password in salary slip

[nareshkannasln]

Closes: #1941

Description: The password policy in Payroll Settings substitutes Employee fields into a template, e.g. SAL-{first_name}-{date_of_birth}. Date fields were interpolated as raw datetime.date objects, which always stringify as ISO, so the password contained 1990-05-08 regardless of the date format configured in System Settings.
An employee whose system is set to dd-mm-yyyy is told the password is their date of birth, tries 08-05-1990, and cannot open the salary slip PDF.

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[greptile-apps]

Confidence Score: 4/5

The change appears safe to merge after the non-blocking documentation omission is addressed.

The date-format regression is covered, and the only accepted issue is the missing contract documentation for the new public helper.

Files Needing Attention: hrms/payroll/doctype/salary_slip/salary_slip.py

_{Reviews (1): Last reviewed commit: "test(salary-slip): add password uses sys..." | Re-trigger Greptile}

[greptile-apps]

Document the date-format helper

format_dates_in_system_format is a new public helper without a docstring, leaving its system-format conversion contract implicit for future callers and maintainers.

Suggested change

	def format_dates_in_system_format(value):
	def format_dates_in_system_format(value):
	"""Wrap date values so string interpolation uses the system format."""

Context Used: Guidelines for reviewing Frappe Framework applicat... (source)

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