Extend BruteForce search to non-XYZ point types

[jeevan6996]

Summary

• add PointRepresentation support to BruteForce search
• keep the existing fast XYZ distance path when the representation is trivial 3D
• use full DefaultFeatureRepresentation dimensions for descriptor point types where available
• allow autoSelectMethod to fall back to BruteForce when FLANN/Nanoflann are unavailable
• add focused BruteForce tests for custom representations, FPFH descriptors, and sparse invalid descriptors

Fixes #6421.

Validation

• configured a focused local build with search/tests enabled
• built target: cmake --build /private/tmp/pcl-bruteforce-build-6 --target test_brute_force_search
• ran: ctest --test-dir /private/tmp/pcl-bruteforce-build-6/test -R brute_force_search --output-on-failure
• ran: git diff --check