fix(useInfiniteScroll): improve promise handling and add flush post to watch

[nhquyss]

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Description

Follow-up improvements to #5110

For detailed, see this comment in PR

Changes

• Move promise check to early return to prevent unnecessary logic execution

• Add flush: 'post' to watch to ensure it runs after DOM updates

Additional context

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commit: 81dda09

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.70%. Comparing base (3dc2d83) to head (1f065a2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5122      +/-   ##
==========================================
- Coverage   67.71%   67.70%   -0.01%     
==========================================
  Files         367      367              
  Lines       14721    14719       -2     
  Branches     2457     2455       -2     
==========================================
- Hits         9968     9966       -2     
  Misses       4742     4742              
  Partials       11       11              

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