fix: add input validation for dataset path — show helpful error for missing files (#118)

[fazalpsinfo-cmyk]

Fix: Add input validation for dataset path with helpful error message

When a user specifies a dataset path that doesn't exist, the error message was a generic DatasetNotFoundError. Now it shows:

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

Changes

1. openagent_eval/datasets/base.py — Enhanced _validate_path() to include current working directory and usage tip in the error details.
2. openagent_eval/datasets/factory.py — Added early path validation in _get_loader() for fail-fast behaviour before loader selection.

Fixes #118

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