Add required-column validation to the Python CLI

[eneskemalergin]

Summary

The Python CLI assumes the expected Spectronaut or MSFragger columns are present. If the wrong export is provided, the script currently fails with low-level column errors.

Tasks

• define the required columns for each supported source
• validate the input table immediately after loading it
• print a clear missing-column error message

Acceptance criteria

• the Python CLI fails early with a readable schema error
• the error lists the missing columns for the selected source

Why this matters

This is one of the most likely user-facing failure modes.