ENH: Add is_split_graph and is_complete_split_graph functions with tests

[sourabh-sudesh-paradeshi]

Summary

This PR introduces two functions to recognize split graphs in networkx.algorithms:

• is_split_graph(G) — returns True if the graph is both chordal and its complement is chordal.
• is_complete_split_graph(G) — returns True if the graph can be partitioned into a clique and an independent set, with all vertices in the independent set adjacent to every vertex in the clique.

Motivation

Split graphs form a well-studied class of graphs that intersect properties of chordal graphs and co-chordal graphs. The recognition functions are useful for research, teaching, and applications involving graph decompositions and forbidden subgraph characterizations.

References

• https://en.wikipedia.org/wiki/Split_graph

[rossbar]

Gentle ping @sourabh-sudesh-paradeshi

[sourabh-sudesh-paradeshi]

Gentle ping @sourabh-sudesh-paradeshi

Refined the logic and tests, all checks pass now — ready for your review. Thanks!