add more coverage for yaml

[derrickaw]

First of several PRs for #39125

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[derrickaw]

/gemini review

[gemini-code-assist]

Code Review

This pull request significantly increases test coverage across several Apache Beam YAML modules, including enrichment, I/O (BigQuery, PubSub, TFRecord), mapping (JavaScript, SQL, and validation), machine learning providers, and various provider utility classes. The only feedback is to remove the duplicated test methods test_flatten_single_pcoll and test_flatten_empty in yaml_provider_unit_test.py.

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