feat: add LocalFileTaskResultStore for caching task results locally

[afarntrog]

Description

Add LocalFileTaskResultStore, a task result store backed by local JSON files that caches EvaluationData per case. This enables persisting and reloading task results across experiment runs without requiring an external data store.

• Implement LocalFileTaskResultStore with save() and load() methods using Pydantic JSON serialization
• Auto-create the storage directory on initialization
• Export the class from the strands_evals package __init__.py

Related Issues

Documentation PR

Type of Change

New feature

Testing

Added unit tests in tests/strands_evals/test_local_file_task_result_store.py covering:

• Round-trip save and load preserves all EvaluationData fields

• Loading a nonexistent case returns None

• Constructor creates nested directories automatically

• Saved files contain valid JSON with expected content

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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