bench(transaction): align raw-get metric with safe path; assert mdbx_get success

[ANtutov]

Reason:

• The safe benchmark (txn.get::(...)) measures value length, while the raw benchmark summed key length, producing inconsistent metrics.
• Without checking the return code, the raw path could silently mis-measure on failures.

Goal:

• Ensure apples-to-apples comparison between safe and raw implementations.
• Improve reliability of the benchmark by catching retrieval errors early.