Explicitly call #to_a to avoid potential GC bug

What does this MR do and why?

We've seen a small number of seg faults that look to be coming from this method. One candidate for a root cause is a known issue in Ruby's GC with Enumerable#flat_map (https://bugs.ruby-lang.org/issues/18140) because the concatenated array can be GC'd prematurely. By calling #to_a explicitly, we insert it into the Ruby stack so that GC can recognize it and keep it around.

There is a fix and backport for this in Ruby, but it may be a while before they're released.

h/t to @stanhu for identifying this as a possible root cause and supplying a solution.

Related fix in another library: https://github.com/pganalyze/pg_query/pull/227

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Related to #352670