Shim to call DeepMerge.deep_merge/3 with a 3-arity resolver without Dialyzer false positives.

DeepMerge's typespec advertises a 2-arity resolver (any(), any() -> any()) but the actual runtime implementation calls the resolver with 3 arguments (key, left, right).

This shim hides the call from Dialyzer while providing the correct typespec for our usage.