Pin default Hugging Face dataset revision

[emmepra]

Summary

Changes

• Pins the default Hugging Face dataset revision used by the Inspect loader and task.
• Documents the default revision and keeps explicit dataset_revision overrides available.
• Adds regression coverage for the loader and task defaults.

Verification

• uv run --python 3.11 --extra test python -m compileall ahb_eval scripts src tests
• uv run --python 3.11 --extra test python -m pytest
• uv run --python 3.11 --extra test inspect eval src/ahb_inspect/tasks.py@ahb --model mockllm/model --model-role grader=mockllm/model --limit 1 --no-log-realtime
• uv run --python 3.11 --extra test inspect eval ahb_inspect/ahb --model mockllm/model --model-role grader=mockllm/model --limit 1 --no-log-realtime