Add input validation for dataset path — show helpful error for missing files

[himanshu231204]

Description

When a user specifies a dataset path that doesn't exist, the error message is a generic file-not-found error. A more helpful message could suggest checking the path and showing the current working directory.

Current Behavior

\
FileNotFoundError: [Errno 2] No such file or directory: 'data/my-dataset.json'
\\

Expected Behavior

\
Error: Dataset file not found at 'data/my-dataset.json'
Checked in: /home/user/project/
Tip: Use an absolute path or check that 'data/my-dataset.json' exists relative to your current directory.
\\

Files to Look At

• \openagent_eval/datasets/base.py\ or \openagent_eval/datasets/factory.py\ — where dataset files are opened
• \openagent_eval/config/validator.py\ — or where config validation happens

Good First Issue ✅

A user-friendly improvement that makes debugging easier for everyone.