Function lookupValidator

Look up the MethodValidators for a dotted method path (e.g. "math.grade1.add") from a validators object that mirrors the API shape.

The validators object has the same nesting as the API:

{ math: { grade1: { add: { input: ..., output: ... } } } }
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This function walks down the path segments and returns the leaf node if it looks like a MethodValidators (has input or output key). Returns undefined if the path doesn't exist or lands on a namespace node (not a leaf with schemas).