Centralize BigQuery Avro loads in BqUtils

[kmontemayor2-sc]

What

• Adds BqUtils.load_files_to_bq for loading one or more GCS files with a caller-provided BigQuery job configuration.
• Preserves synchronous and asynchronous load behavior while centralizing load timing and result logging.
• Routes embedding and prediction Avro exports through BqUtils and uses BqUtils.join_path for the destination table.
• Adds focused unit coverage for the utility and export delegation, plus end-to-end prediction export coverage.

Why

The export helpers maintained their own BigQuery client and duplicated load-job orchestration. Centralizing that behavior in BqUtils gives file-based loads the same client ownership and a reusable sync/async API while preserving existing public export signatures and load semantics.

Testing

• bq_test.py: 9 tests passed.
• Export delegation tests: 4 tests passed.
• Scoped ty checks passed for all changed Python files.
• Ruff check and formatting passed for all changed Python files.

[mkolodner-sc]

Thanks Kyle!