fix: handle empty request timing lists and complete compute_metrics in get_numbers_sequential (closes #542)

[botbikamordehai2-sketch]

What

The get_numbers_sequential() function in aios/utils/calculator.py had two bugs:

1. It accessed request_waiting_times[0] and request_turnaround_times[0] without checking if the lists were empty, causing an IndexError.
2. The compute_metrics helper function at the end of get_numbers_sequential was incomplete/truncated, and the function had no return statement.

Fix

1. Added guard checks (if request_waiting_times: and if request_turnaround_times:) before accessing index 0 of the timing lists.
2. Completed the compute_metrics function body and added the return metrics statement, mirroring the pattern already used in get_numbers_concurrent.

Closes #542