reactivity: run $effect immediately after async derived resolves (non-suspend path)(#16497)

[Jeelislive]

[Bug] Fixes issue where $effect does not run immediately after an async derived resolves unless another “wakeup” occurs.

[Change] In async_derived()’s handler, when the derived did not suspend and the boundary is not pending, explicitly flush and deactivate the current batch so user effects run right away.

[modified] packages/svelte/src/internal/client/reactivity/deriveds.js
In async_derived.handler:

- After internal_set(signal, value), if !should_suspend && !pending, call:
    - batch.flush();
    - batch.deactivate();

Behavior before vs after:

• Before: $effect runs only after a subsequent wake/wakeup following async derived resolution.
• After: $effect runs immediately after the async derived resolves in the non-suspend path.

Related

• Fixes: $effect does not run after async derived completes, but after the next wakeup #16497

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