Shared::operator bool returns the opposite of handle validity

[jeet-dekivadia]

I noticed that Shared::operator bool() appears to be inverted:

Awaitable* get() const;
explicit operator bool() const { return !state_; }

That makes a default-constructed Shared evaluate true, while a constructed Shared with a valid state evaluates false. This is the opposite of the surrounding handle-like API: get() returns nullptr for the empty case, closed() treats an empty Shared as closed, and IntrusivePtr itself uses ptr_ != nullptr for its boolean conversion.

A small example of the observable behavior:

auto shared = Shared([]() -> Task<int> { co_return 42; });
CATCH_CHECK(shared); // currently fails

decltype(shared) empty;
CATCH_CHECK(!empty); // currently fails

I have a small fix with a regression in the existing shared tests.

[jeet-dekivadia]

I opened #27 with the small fix and a regression in the existing shared adapter test.