Expand CPU quantile property tests and remove redundant cases

[RAMitchell]

Summary

This PR continues the CPU quantile test rewrite by expanding the property-style coverage in
test_quantile.cc and removing redundant legacy CPU quantile tests.

The new CPU property suites now cover:

• summary-level sketch invariants
• container-level cut invariants
• SketchOnDMatrix wrapper invariants
• distributed row-split and column-split properties

What Changed

• broaden the shared CPU quantile case matrix in tests/cpp/common/test_quantile_helpers.h
• add local cut validators used by the new property tests
• replace legacy distributed identity-style tests with property checks against reference data
• remove redundant CPU quantile tests from test_quantile.cc and test_hist_util.cc

Testing

• cmake --build build-cpu --target testxgboost -j35
• focused CPU quantile slice

Result:

• 140/140 passed

[Copilot]

Pull request overview

Expands the CPU quantile test rewrite by adding property-style invariants for summary/container/sketch paths (including distributed row-/col-split) and removing redundant legacy CPU quantile tests.

Changes:

• Refactors summary invariants into a shared helper and adds container-level cut validators (numerical + categorical).
• Adds new CPU property suites covering HostSketchContainer, SketchOnDMatrix, and distributed row-/column-split behavior.
• Removes legacy CPU quantile/hist-util tests that are now covered by the new property checks.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tests/cpp/common/test_quantile.cc	Adds new property-based test suites for summaries/cuts (local + distributed), and removes redundant legacy tests.
tests/cpp/common/test_quantile_helpers.h	Introduces container-case matrix, reference-column tooling, and cut validation helpers used by the new tests.
tests/cpp/common/test_hist_util.cc	Removes redundant CPU sketch/cut accuracy tests superseded by the new quantile property coverage.

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