fix(eventsub/string): minor cleanups

[Nerixyz]

Followup to #5968.

• We can't make the constructor taking a std::string_view constexpr, because we can store a QString which isn't constexpr → our destructor isn't constexpr.
• The constructor should be a bit simpler now - it shows what we're doing (if we can't fit data in-place, we allocate, and then copy the data)
• view() can be constexpr (not sure what the use is)
• length was never used
• We don't need to clear the storage when moving, because we clear the flags (0 = empty string). In debug mode however, I'd like to make sure it's actually never used, so we put a tombstone value there (if we'd access it, we'd get garbage).
• For clang-20, we need to cast the argument to std::mem* functions to void* explicitly - otherwise, it warns that the type isn't trivially copyable. It doesn't know that (1) we're moving and (2) QString is trivially relocatable (memcpy-able).
• Added some tests to make sure that the underlying QString is actually destructed (similar to chore(tests): try to generate coverage better #5972).