SOLR-18221 Add support for percentile in rollup for streaming expressions

[KhushJain]

https://issues.apache.org/jira/browse/SOLR-18221

Description

Added support for per(col, percentile) metric in rollup() streaming expressions.
The per(col, percentile) metric in rollup() streaming expressions was non-functional; the PercentileMetric class was a stub copy-pasted from MaxMetric that returned garbage values.

Solution

Implemented PercentileMetric using T-Digest (AVLTreeDigest, compression=100), matching the server-side PercentileAgg used by facet() and stats(). Supports decimal percentiles (e.g., per(a_i, 99.9)).

Tests

testPercentileRollupStream: validates per() with multiple percentiles (0, 50, 99.9, 100)

Checklist

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• I have developed this patch against the main branch.
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• I have added tests for my changes.
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