Add buffer validation and tests for passthrough concurrent file merging

[wswsmao]

This PR addresses the outstanding issue discussed in this comment. The following changes are introduced:

1. New checkHoles method
   During the processBatchChunks, we now record the starting position and the actual size of the buffer read each time. This information is then validated by the checkHoles method to ensure there are no gaps or overwrites in the buffer.
2. Validation after w.Write(buffer)
   We also added validation for the return value after w.Write(buffer) to prevent any unknown write errors.
3. Supplementary test cases
   Additional test cases have been included to cover three scenarios: normal batch processing with validation, batch processing with unhandled chunks, and errors in index calculations.

[ktock]

Thanks