fix: await async onSuccess/onError callbacks to prevent unhandled rej…

[shunn2]

Summary

This PR fixes an issue where useSWRMutation does not await async onSuccess or onError callbacks.
Previously, when developers provided an async callback (for example, revalidating data with mutate(key)), it was executed in parallel with the return of trigger(), leading to race conditions and unhandled promise rejections.

Now, async callbacks are awaited safely, preventing unhandled rejections while maintaining backward compatibility.

Example

const { trigger } = useSWRMutation('/api/item', updateItem, {
  onSuccess: async () => {
    await mutate('/api/item') // unhandled rejection if this fails
  }
})

await trigger({ id: 1 })

Fix

• Await onSuccess and onError callbacks inside trigger().
• Wrap both in try/catch to prevent callback errors from propagating.
• Log callback errors to console.error for visibility.
• Preserve existing behavior:
• The mutation result and UI state remain unchanged even if a callback fails.
• No breaking change for synchronous callbacks.

Related Issue: #4171

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