Complete SQL tasks and add automated expense report tests

[radmitqa]

Summary

• Added employee, expense, and invoice-related SQL solutions.
• Added manager cycle detection query.
• Added largest expense report query.
• Added supplier payment plan query.
• Added pytest coverage for the largest expense report.
• Added test dependencies and test execution instructions to the README.

Testing

• python -m pytest -v
• Result: 1 passed

The expense report test validates:

• expected employee
• employee and manager names
• manager ID
• total expense amount
• result set/order
• exclusion of employees below the threshold