Migrate the S3 storage connector's test-connection off the legacy test_connection_steps helper onto the declarative @check / ChecksProvider / ErrorPack framework.

S3 is the first connector of the Storage vertical to migrate, so this also seeds the shared checks/storage.py scaffolding (a StorageStep enum + reusable boto3 helpers) that the remaining storage connectors (gcs, …) will reuse.

Scope

• New core/connections/test_connection/checks/storage.py: StorageStep enum (ListBuckets, GetMetrics) + list_buckets / probe_buckets / list_metrics helpers that return Evidence and raise CheckError on failure.
• S3Checks provider + S3_ERRORS botocore ErrorPack (folding in the shared network pack); S3Connection.checks() replaces the old test_connection override.
• Set category: ConnectionGate on the ListBuckets step in testConnections/storage/s3.json so a failed access gate short-circuits.
• Colocated unit test at ingestion/tests/unit/source/storage/s3/test_connection.py.