Fix infinite loops in graph generation logic

[google-labs-jules]

This change fixes the reported infinite loop bugs in G.graph.

Previously, Bipartite graph generation selected a partition size randomly. If the selected partition was too unbalanced (e.g., 1 node in set A, 9 in set B), it might not support the requested number of edges, causing the rejection sampling loop to run forever. The fix mathematically calculates the valid range of partition sizes that can support m edges and samples from that range.

Additionally, the change adds strict validation for the maximum possible edges for all graph types (DAG, Bipartite, Simple) to ensure that impossible requests fail fast with an error instead of hanging.

The connectivity logic for DAG and Bipartite graphs was also improved to ensure that the initial connected edges respect the specific constraints of those graph types (e.g., edges only between partitions for Bipartite, edges respecting topological sort for DAG).

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PR created automatically by Jules for task 9037485127498503475 started by @yviscool

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