Part of #32191 (Meta-ticket: Audit/fix all uses of MixedIntegerLinearProgram in sage.combinat, sage.graphs).

Here we avoid the use of Integer(round(p.solve(...))) whenever possible.

CC: @mkoeppe @tscrim @dimpase

Component: graph theory

Author: David Coudert

Branch/Commit: a000519

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/32428