Add signed file URL resolver

[creatorrr]

trafficstars

Summary

• generate signed URLs from S3 clients
• expose resolve_url in evaluator
• document referencing files in workflows
• note new feature in CHANGELOG

Testing

• ruff format agents-api/agents_api/clients/sync_s3.py agents-api/agents_api/clients/async_s3.py agents-api/agents_api/common/utils/evaluator.py
• ruff check agents-api/agents_api/clients/sync_s3.py agents-api/agents_api/clients/async_s3.py agents-api/agents_api/common/utils/evaluator.py
• pyright agents-api/agents_api/clients/sync_s3.py agents-api/agents_api/clients/async_s3.py agents-api/agents_api/common/utils/evaluator.py (fails: Import could not be resolved)