feat: add endpoint_url support to s3_sync for MinIO compatibility

[koriyoshi2041]

Summary

Addresses #435.

Add an optional endpoint_url parameter to s3_sync() and ensure_bucket_exists() in art.utils.s3. When not provided, both functions fall back to the AWS_ENDPOINT_URL environment variable. This enables S3-compatible services like MinIO without requiring callers to manage the env var themselves.

Changes

• s3_sync(): new endpoint_url keyword argument, passed as --endpoint-url to the AWS CLI
• ensure_bucket_exists(): same parameter, applied to both the head-bucket check and create-bucket commands
• Both functions fall back to os.environ.get("AWS_ENDPOINT_URL") when the parameter is None
• Also documented the previously undocumented delete parameter in s3_sync()

Usage

# Option 1: Set the env var (works for all code paths)
os.environ["AWS_ENDPOINT_URL"] = "http://minio:9000"

# Option 2: Pass explicitly
await s3_sync(source, dest, endpoint_url="http://minio:9000")

Fully backward-compatible — the parameter defaults to None and all existing callers are unaffected.

Test plan

• Verify s3_sync() includes --endpoint-url when parameter is set
• Verify s3_sync() includes --endpoint-url when AWS_ENDPOINT_URL env var is set
• Verify s3_sync() omits --endpoint-url when neither is set
• Verify ensure_bucket_exists() passes endpoint to all three CLI commands
• uv run prek run --all-files passes