Fix DynamoDB listing assertion

[fallintoplace]

Summary

• Move DynamoDB commit coordinator S3 listing validation into a small helper.
• Fix the missing-Contents assertion so required files fail the test when absent.
• Add focused regression checks for missing Contents, UUID-suffixed staged commit files, and non-json suffixes.
• Run the helper regression checks from the Build GitHub Actions workflow before heavier setup.

Root Cause

The previous assert used assert(condition, message), which creates a truthy tuple in Python and silently disables the failure path.

Matching Notes

The stricter file check now requires the file name to start with the requested zero-padded version and end with .json, while still allowing UUID-suffixed staged commit files such as 00000000000000000001..json.

Validation

• python3 -m py_compile spark/src/main/java/io/delta/dynamodbcommitcoordinator/integration_tests/dynamodb_commitcoordinator_integration_test.py spark/src/main/java/io/delta/dynamodbcommitcoordinator/integration_tests/_dynamodb_commitcoordinator_listing.py
• python3 spark/src/main/java/io/delta/dynamodbcommitcoordinator/integration_tests/_dynamodb_commitcoordinator_listing.py
• cd spark/src/main/java/io/delta/dynamodbcommitcoordinator/integration_tests && python3 _dynamodb_commitcoordinator_listing.py
• git diff --check

Did not run the full AWS/Spark integration test locally.