fix: move import to top-level in bigquery-data-agent test

[eliasecchig]

Summary

• Fix PLC0415 linting error in bigquery-data-agent test file

Changes

• Moved BigQueryAgentAnalyticsPlugin import from inside test function to top-level
• Removed unused pytest import
• Organized imports according to ruff standards

Impact

• Resolves PLC0415 linting error: "import should be at the top-level of a file"
• Improves code organization and follows Python best practices

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