Fix malformed BigQuery PK metadata SQL

[ianu82]

Summary

• remove the trailing comma from the meta_get_primary_keys SELECT list in the BigQuery handler
• extend the unit test to assert the generated query does not include a dangling comma before FROM

Why

The trailing comma produces invalid SQL and breaks metadata introspection for primary keys.

Validation

• python3 -m py_compile mindsdb/integrations/handlers/bigquery_handler/bigquery_handler.py tests/unit/handlers/test_bigquery.py
• pytest -q tests/unit/handlers/test_bigquery.py -k meta_get_primary_keys_filters (fails in this environment due to missing optional dependency google.api_core)

[github-actions]

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[ianu82]

recheck