fix(set_cover): initialize thread_pool_ and remove stale ThreePhase declaration (fixes #5341)

[jdymitarai]

Description

Fixes #5341.

This PR addresses two issues in ortools/set_cover/set_cover_lagrangian:

1. Remove stale ThreePhase declaration:

   • set_cover_lagrangian.h declared void ThreePhase(Cost upper_bound); with no corresponding definition in ortools/set_cover (the three-phase algorithm was migrated to set_cover_cft.cc as RunThreePhase).
   • Calling or binding &SetCoverLagrangian::ThreePhase leads to link-time or runtime symbol import errors (undefined symbol: _ZN19operations_research18SetCoverLagrangian10ThreePhaseEd). The stale declaration is removed.

2. Initialize thread_pool_ to prevent SIGSEGV in ComputeLowerBound:

   • Previously, thread_pool_ was initialized to nullptr in constructors unless UseNumThreads() was explicitly called prior to ComputeLowerBound(). Uninitialized thread_pool_ dereferences resulted in a segmentation fault when parallel methods scheduled tasks.
   • Initialized thread_pool_ with this->params().num_threads in constructors, and added a defensive guard in ComputeLowerBound() ensuring thread_pool_ is initialized if null.

Testing

• Added regression test SetCoverTest.LagrangianComputeLowerBoundDefaultThreadPool in ortools/set_cover/set_cover_test.cc verifying that ComputeLowerBound() executes successfully on a fresh SetCoverLagrangian without requiring UseNumThreads().
• Updated ortools/set_cover/BUILD.bazel to include :set_cover_lagrangian in set_cover_test dependencies.