refactor: Use C++20 'requires' in ActivityResult constructor (#1420)

## Summary

**What is the goal of this PR?**

Replace SFINAE std::enable_if_t with a C++20 `requires` clause for
clearer constraint expression and better compiler diagnostics on
mismatch.

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src/activities/ActivityResult.h

@@ -59,7 +59,8 @@ struct ActivityResult {
 
   explicit ActivityResult() = default;
 
-  template <typename ResultType, typename = std::enable_if_t<std::is_constructible_v<ResultVariant, ResultType&&>>>
+  template <typename ResultType>
+    requires std::is_constructible_v<ResultVariant, ResultType&&>
   // cppcheck-suppress noExplicitConstructor
   ActivityResult(ResultType&& result) : data{std::forward<ResultType>(result)} {}
 };