Add the executor-level seam missing from bf-14wgq: mockito-driven unit tests
for `ReshardExecutor::compute_source_document_count`, which delegates to the
shared `index_stats::aggregate_index_stats`. The pure reduction policy is
already covered directly; these exercise the real reqwest transport against
`/indexes/{uid}/stats` on mock nodes returning known per-node counts:
(a) multiple healthy nodes with differing counts → result is the max,
not the sum (each address hosts a full replica);
(b) one node returning HTTP 5xx → logged and skipped, max of the rest;
(b') a node 404-ing (absent replica) → counts as zero, not a failure;
(c) every node failing → denominator falls back to 0.
Adds `mockito = "1"` as a miroir-core dev-dependency, mirroring how the
miroir-proxy ILM acceptance tests drive the same endpoint. Test-only change;
no production code modified.
Co-Authored-By: Claude <noreply@anthropic.com>
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