Add property-based CPU quantile summary tests

[RAMitchell]

Summary

This PR starts restructuring CPU quantile tests around property-style invariants instead of bespoke one-off cases.

The first step focuses on the summary layer in tests/cpp/common/test_quantile.cc:

• add a shared quantile test helper header for deterministic case generation and reference rank checks
• add anchor and sampled summary cases that exercise both exact-support and compressed regimes
• validate summary-level invariants directly, including sorted unique support, exact retention when the support fits in the target bin count, and the summary query rank bound
• keep the existing container/distributed tests in place for now
• remove the old SetPruneInplace summary-only test

This is intended as the first step toward replacing the older ad hoc quantile tests in test_quantile.cc with layered property-based tests.

Testing

Ran locally:

• ./build-cpu/testxgboost --gtest_filter='*QuantileSummary*'
• ./build-cpu/testxgboost --gtest_filter='Quantile.*'

[Copilot]

Pull request overview

Restructures CPU quantile summary tests toward property-based invariants by introducing shared deterministic case generation helpers and adding new invariant-focused summary tests.

Changes:

• Added test_quantile_helpers.h with deterministic summary case generation and reference/rank-check utilities.
• Added new parameterized summary tests validating support monotonicity, exact-retention when representable, and rank-error bounds.
• Removed the legacy SetPruneInplace summary-only test.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/cpp/common/test_quantile.cc	Adds new property/invariant-based quantile summary tests and removes the old prune-inplace test.
tests/cpp/common/test_quantile_helpers.h	Introduces shared utilities for generating deterministic summary inputs and computing reference/rank-error checks.

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