Fix edge case in build_data_parallel_buffer_index and improve clarity

[shjwudp]

What does this PR do ?

Fix edge case in build_data_parallel_buffer_index and improve clarity

• Resolve a bug in handling edge cases with a large number of fragments.
• Refactor build_data_parallel_buffer_index for clearer structure and better maintainability, adding inline documentation.
• Add comprehensive unit tests for parameter splitting and buffer index construction to verify correctness.

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[shjwudp]

/ok to test 07e17d3

[shjwudp]

/ok to test 2c98a29

[cspades]

@shjwudp This PR looks pretty important, do you still want to push it through? Or is it an edge case that can be updated in the rewrite?