fix: make _ToProc interface more specific

Narrow the handler interface signature from `(untyped) -> untyped`
to `(Hash[Symbol, untyped]) -> void` since event handlers always
receive event details hashes and their return values are unused.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Jose Colella <jose.colella@gusto.com>

Author: josecolella

sig/open_feature/sdk/event_dispatcher.rbs

@@ -18,7 +18,7 @@ module OpenFeature
     end
 
     interface _ToProc
-      def call: (untyped) -> untyped
+      def call: (Hash[Symbol, untyped]) -> void
     end
   end
 end