Add unit tests for hierarchical all_reduce and all_gather

[iemAnshuman]

Tests verify correctness of the reduce+broadcast and gather+broadcast compositions at varying site counts (2-64) and arities (2, 4, 8, 16). Both local (single-locality, multi-threaded) and distributed (multi-locality) configurations are covered.

Follows the same pattern as the existing reduce_hierarchical, gather_hierarchical, broadcast_hierarchical, and scatter_hierarchical tests.

Companion to #7160 which added the hierarchical all_reduce and all_gather implementations.

Proposed Changes

• Add all_reduce_hierarchical.cpp unit test
• Add all_gather_hierarchical.cpp unit test
• Register both in CMakeLists.txt under the networking test suite

Any background context you want to provide?

The hierarchical all_reduce and all_gather were merged in #7160 but without dedicated unit tests. These tests fill that gap.

Checklist

• I have added a new feature and have added tests to go along with it.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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[StellarBot]

Can one of the admins verify this patch?

[iemAnshuman]

@hkaiser CMakeLists sorting fixed per your review. All CI green. Ready for re-review.

[hkaiser]

LGTM, thanks!