Bug in ParentJoin Benchmark Recall

[vigyasharma]

While working on apache/lucene#14173, I discovered that the parentJoin benchmark yields different recall results with merging enabled v/s disabled. Merging should not really have an impact on recall. We are likely doing something wrong in comparing the exact and approximate search results in the benchmark.

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# parentJoin (on main)
## Results:
recall  latency (ms)    nDoc  topK  fanout  maxConn  beamWidth  quantized  index s  index docs/s  force merge s  num segments  index size (MB)  vec disk (MB)  vec RAM (MB)
 0.958         1.160   10000   100      50       32        100         no     1.49       6706.91           1.85             1            29.67         29.297        29.297
 0.925         2.392  100000   100      50       32        100         no    34.98       2858.86           7.86             1           297.91        292.969       292.969
 0.914         2.972  200000   100      50       32        100         no    63.80       3134.94          43.48             1           596.14        585.938       585.938
 0.904         4.292  500000   100      50       32        100         no   151.49       3300.57         147.08             1          1491.81       1464.844      1464.844

## parentJoin (on main, merge disabled):
## Results:
recall  latency (ms)    nDoc  topK  fanout  maxConn  beamWidth  quantized  index s  index docs/s  force merge s  num segments  index size (MB)  vec disk (MB)  vec RAM (MB)
 0.440         1.297   10000   100      50       32        100         no     1.76       5694.76           2.03             1            29.67         29.297        29.297
 0.692         2.596  100000   100      50       32        100         no    11.35       8807.47          29.76             1           297.86        292.969       292.969
 0.530         3.173  200000   100      50       32        100         no    22.03       9077.71          67.91             1           596.24        585.938       585.938
 0.598         4.368  500000   100      50       32        100         no    53.20       9398.50         204.29             1          1493.26       1464.844      1464.844