refactor(bigtable): consolidate integration tests

[google-labs-jules]

This change refactors the Bigtable integration tests to reduce redundancy and improve performance. The TestIntegration_Read and TestIntegration_FullReadStats tests have been merged into a single test, TestIntegration_ReadStats, which now verifies both the read results and the read statistics. This eliminates a redundant set of table operations and speeds up the test suite. Additionally, a bug in the reverse scan test cases that was introduced during the refactoring has been fixed.

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PR created automatically by Jules for task 15992946186742310529 started by @bhshkh

[google-labs-jules]

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