Update timestamp output format system test to use direct request

[danieljbruce]

The system test for timestamp output format was modified to use a direct API request via the ServiceObject.request method inherited by the Table class. This change avoids dependency on unreleased library features in @google-cloud/bigquery and allows for direct verification of high-precision timestamp strings as returned by the BigQuery API.

Key changes:

• Replaced table.getRows(options) with (table as any).request({ uri: '/data', qs: options }, callback).
• Updated assertions to validate the raw API response structure (resp.rows[0].f[0].v).
• Wrapped the callback-based request in a Promise to maintain compatibility with the existing async/await test structure.
• Handled both success and error cases as defined in the test suite.

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PR created automatically by Jules for task 3432412269925724150 started by @danieljbruce

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