coalesce updates to reduce intermediate updates

[zhaohuabing]

fix: #7308

In my test setup, I added a 5-second sleep to mimic a slow process, and then rolled out the backend deployment from 1 to 100 replicas. The result looks good:

2025-10-24T10:10:11.791Z	INFO	watchable	message/watchutil.go:144	coalesced updates	{"runner": "gateway-api", "count": 1, "before": 81}
2025-10-24T10:10:16.239Z	INFO	watchable	message/watchutil.go:144	coalesced updates	{"runner": "gateway-api", "count": 1, "before": 10}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.08%. Comparing base (f133f5e) to head (b1a7508).
⚠️ Report is 58 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7328      +/-   ##
==========================================
+ Coverage   72.06%   72.08%   +0.01%     
==========================================
  Files         230      230              
  Lines       33404    33424      +20     
==========================================
+ Hits        24074    24095      +21     
  Misses       7583     7583              
+ Partials     1747     1746       -1     

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[arkodg]

LGTM thanks