refactor: make to_optimization_problem a free function

[ramakrishnap-nv]

2 of 4 toward a CUDA-free client library. Stacked on #1801 — review that first; the base will move to main once it lands.

The problem

to_optimization_problem() was a virtual member on optimization_problem_interface_t, so it occupied a slot in the vtable of every implementer — including cpu_optimization_problem_t, whose vtable therefore held an entry only libcuopt can define.

Vtable relocations are resolved eagerly at load time, unlike ordinary function calls. So this cannot be deferred, hidden behind lazy binding, or worked around with a dispatch hook: any library carrying that vtable is unloadable without libcuopt.so present.

The change

Now a free function declared in optimization_problem.hpp, defined in cpu_optimization_problem_to_gpu.cpp, dispatching on the concrete type:

-  auto gpu = problem->to_optimization_problem(&handle);
+  auto gpu = to_optimization_problem(*problem, &handle);

The GPU override was a one-line return nullptr ("already a GPU problem"), so the dispatch is a single dynamic_cast and semantics are unchanged — a GPU-backed problem still yields nullptr. cpu_optimization_problem_t befriends the function to reach its host-side storage.

6 call sites updated: pdlp/solve.cu, mip_heuristics/solve.cu, grpc/server/grpc_worker.cpp, and two in solution_interface_test.cu.

Bonus

Moving the definition to its own TU also keeps <optimization_problem.hpp> and the raft handle out of cpu_optimization_problem.cpp, which is otherwise pure host code.

Discussion point

This trades a virtual for a dynamic_cast. That's the deliberate choice — the alternative (keeping it virtual and dispatching through a registered function pointer) doesn't work, because the vtable slot still needs a definition at load time. Happy to discuss if there's a third option I've missed.

Testing

Full build + 126 test binaries, 0 errors. 111/125 pass; the 14 failures are cudaErrorUnknown from a locally wedged nvidia_uvm, identical on unmodified main.

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[ramakrishnap-nv]

/ok to test

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