Return neighbors in a deterministic order

[juan52878911]

Summary

PyGraph.neighbors, PyDiGraph.neighbors and PyDiGraph.neighbors_undirected returned neighbor node indices in a non-deterministic order that could change between runs. This makes them return a stable, deterministic order while keeping their multigraph de-duplication behaviour.

Fixes #1501.

Root cause

All three methods deduplicated neighbors by collecting into a HashSet<usize> and then draining it:

.map(|node| node.index())
.collect::<HashSet<usize>>()
.drain()
.collect(),

HashSet's iteration order is randomized, so the order of the returned indices was arbitrary and unstable — it could differ between runs and even between successive calls on the same graph.

Fix

We swapped HashSet with IndexSet

Tests

Added regression tests in tests/graph/test_neighbors.py and tests/digraph/test_neighbors.py that assert a stable order across repeated calls, the exact expected order, and correct de-duplication of parallel edges.

Verified locally against a fresh build (rustworkx main): the new tests pass, the full tests/graph (820) and tests/digraph (918) suites pass, and cargo fmt/cargo clippy -- -D warnings are clean.

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