fix: merge test/test-full leaf variants per-cell instead of wholesale

[cgoea]

Motivation

upsert_leaf's test/test-full path replaced a per-arch test leaf (and its full variants list) wholesale whenever the do-not-downgrade guard accepted the update. The build phase already avoided this via _merge_variant_leaf, which merges matrix cells one at a time, but test/test-full had no equivalent.

Technical Details

• Added merge_matrix_test_leaf(existing, new) to therock_status_document.py, mirroring _merge_variant_leaf.
• Wired StatusDocument.upsert_leaf's test/test-full branch: when the incoming leaf carries variants, it now merges into the existing leaf via merge_matrix_test_leaf and always reports the write as accepted.
• Wired _refresh_same_run_fanout_tests in therock_update_status_json.py to merge cell-by-cell via merge_matrix_test_leaf instead of overwriting existing.variants = leaf.variants wholesale.

Test Plan

• Updated test_upsert_test_replaces_whole_leaf_with_variants_atomically and renamed test_upsert_test_with_variants_rejects_lower_attempt_wholesale → test_upsert_test_with_variants_rejects_lower_attempt_per_cell
• Added test_upsert_test_with_variants_always_returns_true, mirroring the existing test_upsert_build_with_variants_always_returns_true build-side coverage.
• Added test_upsert_test_merge_does_not_regress_completed_cell: the core push-race regression test

Test Result

All tests passed.