Complete Graphs.jl AbstractGraph API for IGraph

[nenadilic84]

Summary

This PR completes the Graphs.jl AbstractGraph interface for the IGraph type, enabling IGraph to be used with all existing Graphs.jl algorithms that operate through the API.

Changes

New interface methods:

• Graphs.edges — lazy IGraphEdgeIterator over igraph edge IDs (0→1 index conversion)
• Graphs.outneighbors / Graphs.inneighbors — via LibIGraph.neighbors with sorted output
• Graphs.add_edge!(g, s, d) — with duplicate-edge and self-loop guards
• Graphs.add_vertex! — via raw igraph_add_vertices ccall
• Graphs.rem_vertex! — via igraph_delete_vertices with igraph_vss_1 selector
• Base.copy — via LibIGraph.copy into uninitialized IGraph

Bug fix:

• Graphs.has_edge now correctly converts 1-based Julia indices to 0-based igraph indices before calling get_eid

Tests:

• Added GraphsInterfaceChecker.jl as a test dependency
• New test file validates both mandatory and optional (mutation) parts of the AbstractGraph interface
• Additional explicit tests for edges roundtrip, neighbors, has_edge, and mutation operations
• All 305 tests pass (up from 267)

Motivation

This addresses part of #446: "Complete Graph API support for the IGraph types (so that IGraph types can be used in all already existing Graph.jl algorithms that do not peek behind the API)" and "use GraphsInterfaceChecker.jl in the test suite".

[codecov]

Codecov Report

❌ Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 6.37%. Comparing base (d37f19c) to head (ebd6464).

Files with missing lines	Patch %	Lines
src/graph_api.jl	97.14%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #35      +/-   ##
=========================================
+ Coverage    5.29%   6.37%   +1.08%     
=========================================
  Files           8       8              
  Lines        4310    4342      +32     
=========================================
+ Hits          228     277      +49     
+ Misses       4082    4065      -17     

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[nenadilic84]

Added CHANGELOG.md entry in the latest push.

Note: several CI checks fail on this PR but they are all pre-existing issues unrelated to this PR's changes:

• Documentation: docs/make.jl does not exist in the repo yet
• Spell Check: .typos.toml config file is missing from the repo
• evaluate: SnoopCompile FieldError on Core.Binding (tooling compatibility issue)
• performance-tracking: BenchmarkCI JSON parse error
• JET: "Bad include call" from include(modifymodule, "LibIGraph.jl") pattern
• Julia alpha: JET incompatible with Julia 1.13 beta

All actual test jobs pass on all platforms (ubuntu, macOS x64/aarch64, windows) and on Julia 1.10.

[nenadilic84]

/claim #446