[C++] UpdateLevelHistogram produces incorrect histograms in fast path #50778

Describe the bug, including details regarding any error messages, version, and platform.

parquet::UpdateLevelHistogram produces incorrect histograms when max_level == 1 and the input contains more than 16,384 levels.

The fast path processes input in chunks, but each iteration accumulates from levels.begin() instead of the current chunk iterator. As a result, the first chunk is counted repeatedly and later chunks are skipped.

Component(s)

C++