fix(core): Fix immediate cancellation of fetch requests

[kitten]

Follow-up to #3043

Summary

The async generator implementation has regressed on the previous one since it doesn't include a delay that respects immediate teardown signals. This is because no yield was present after the initial microtick and before the fetch call.

I'm still trying to find a more elegant solution to this, but it doesn't seem possible to yield without emitting a value, and async generators may not abort on await statements, which seems counterintuitive.

Changeset hasn't been added since it's a follow-up fix.

Set of changes

• Add yield after microtick delay to allow fetchOperation to abort early
• Update tests to correctly detect missing early-terminations