fix: Infinite subgraph_isomorphisms_iter for empty isomorphisms - #780

…tgraph#780)

The newly added test fails with the old code, since initial state is
always complete and hence the iterator's return value will always be
`Some(_)`.

I fixed this by making `GraphMatcher` the only entrypoint to
`isomorphisms`, and moving that edge case logic into its `Iterator`
implementation. This is a bit awkward, but I am unsure how to adapt
`isomorphisms` itself, since it currently assumes having at least one
mapped node pair before returning a result.

Benchmark comparisons for the changed code:
<details><summary>Details</summary>
<p>
Command: `cargo +nightly bench --bench iso`

Old:
```
test full_iso_bench            ... bench:         950.78 ns/iter (+/- 77.47)
test petersen_iso_bench        ... bench:         643.20 ns/iter (+/- 47.23)
test petersen_undir_iso_bench  ... bench:         508.70 ns/iter (+/- 29.86)
test praust_dir_no_iso_bench   ... bench:     590,866.19 ns/iter (+/- 23,428.15)
test praust_undir_no_iso_bench ... bench:     602,564.95 ns/iter (+/- 52,031.86)
```

New:
```
test full_iso_bench            ... bench:         934.26 ns/iter (+/- 52.24)
test petersen_iso_bench        ... bench:         622.69 ns/iter (+/- 24.05)
test petersen_undir_iso_bench  ... bench:         498.76 ns/iter (+/- 35.59)
test praust_dir_no_iso_bench   ... bench:     591,756.12 ns/iter (+/- 33,682.87)
test praust_undir_no_iso_bench ... bench:     620,409.33 ns/iter (+/- 27,801.96)
```

</p>
</details>

## 🤖 New release

* `petgraph`: 0.8.2 -> 0.8.3 (✓ API compatible changes)

<details><summary><i><b>Changelog</b></i></summary><p>

<blockquote>

##
[0.8.3](https://github.com/petgraph/petgraph/compare/[email protected]@v0.8.3)
- 2025-09-28

### Bug Fixes

- Infinite `subgraph_isomorphisms_iter` for empty isomorphisms
([#780](#780))
- Algos don't work on `UndirectedAdaptor`
([#870](#870))
([#871](#871))
- use a queue for SPFA
([#893](#893))
- `StableGraph::reverse` breaks free lists
([#890](#890))

### Documentation

- Fix examples link in README and unify typesetting of one word
([#823](#823))
- Add link to multigraph definition to isomorphism algos
([#824](#824))
- Fix auxiliary space (and time) complexity of bron-kerbosch
([#825](#825))
- Fix Typo in Operator Module Documentation
([#831](#831))
- Sync the crate feature flags in the README and docs
([#832](#832))
- Remove all \[Generic\] tags from algo docstrings
([#835](#835))
- Fix typos in comments
([#836](#836))
- Revamp CONTRIBUTING.md
([#833](#833))
- Update `GraphMap` link in README
([#857](#857))
- Add doc comment for `Dot::with_attr_getters`
([#850](#850))
- Specify iteration order for neighbors and edges and their variants
([#790](#790))
- Collection of Doc fixes
([#856](#856))

### New Features

- Add `into_nodes_edges_iters` to `StableGraph`
([#841](#841))
- Add methods to reserve & shrink `StableGraph` capacity
([#846](#846))
- Add Dinic's Maximum Flow Algorithm
([#739](#739))
- make Csr::from_sorted_edges generic over edge type and properly
increase edge_count in Csr::from_sorted_edges
([#861](#861))
- Add `map_owned` and `filter_map_owned` for `Graph` and `StableGraph`
([#863](#863))
- Add dijkstra::with_dynamic_goal
([#855](#855))
- Fix self-loop bug in all_simple_paths and enable multiple targets
([#865](#865))
- mark petgraph::dot::Dot::graph_fmt as public
([#866](#866))
- Add bidirectional Dijkstra algorithm
([#782](#782))

### Performance

- Make A* tie break on lower h-values
([#882](#882))

### Refactor

- add examples for scc algorithms and reorganize into dedicated module
([#830](#830))
- Remove unnecessary trait bounds from impls/methods
([#828](#828))
- replace uses of 'crate::util::zip' with 'core::iter::zip'
([#849](#849))
- Fix clippy (and other) lints
([#851](#851))
- Cleanup repo ([#854](#854))
- replace crate::util::enumerate with Iterator::enumerate
([#881](#881))

### Testing

- Add dependency list for 'quickcheck' feature
([#822](#822))
- Fix feature cfg capitalization in doctest
([#852](#852))
</blockquote>


</p></details>

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Egor Starovoitov <[email protected]>