[BUG] Fix CAGRA graph optimization bug

[enp1s0]

The merging process of the pruned and revedge graphs is inappropriate, resulting in neighbor index duplication. This PR fixes the issue and adds the validation of graph indices at the end of the graph::optimze function.

The simplest solution to fix this issue is 1151511, but it ignores the memory space optimization by rapidsai/raft@12480cf. The code changes in this PR apply the memory space optimization to the simplest solution to remove the memory space for the pruned graph (pruned_graph).

This PR also fixes a problem that throwing an exception from an OMP loop does not work as expected.

[enp1s0]

The new optimize function gets stuck in cagra_extreme_inputs_oob_test because there are some invalid index (~0u) nodes in the initial knn graph. It would be better to check the invalid nodes first in the function and abort the process if there are any.

It seems too severe to abort the optimization process if there is even one invalid node, so I updated the code not to abort unless the pruned graph can be generated even if there are invalid nodes in the initial kNN graph.

[enp1s0]

@anaruse Can you review the update? It is related to merging the pruned graph and MST optimization edges.