Fix: Evaluator contract-file input

[mokita-j]

This PR refines the validation logic for the --contract-file input argument in the evaluator.

• The argument now only accepts paths to Solidity contracts that exist under smartbugs-curated/0.4.x/contracts/dataset.
• Accepted values must follow the relative format: <vulnerability-type>/<filename>
• Any inputs that do not match this pattern or do not correspond to an existing contract are excluded from patching and exploit testing.

This ensures the evaluator only processes valid dataset contracts and prevents errors caused by invalid paths.