fix: confirm NotFound against the API server before dropping status writes

[zhaohuabing]

Description

Fixes #9536: under high watch-event churn (e.g. ~300+ HTTPRoutes on a single Gateway with ArgoCD auto-sync), some objects end up with an empty status ({"parents":[]}) forever, even though the data plane is programmed correctly. A controller restart flushes all of them at once.

Root cause

The status updater's apply() reads the object with the manager's cache-backed client before writing:

if err := u.client.Get(ctx, update.NamespacedName, obj); err != nil {
    if kerrors.IsNotFound(err) {
        return nil // silently skipped, and counted as a success
    }
    return err
}

Under churn the informer cache lags behind the API server, so this Get can return NotFound for a freshly-created object the cache hasn't caught up to yet. apply() then silently returns, so the status is never written.

Fix

When the cache-backed Get returns NotFound, confirm against an uncached APIReader (mgr.GetAPIReader()) before giving up. If the object is genuinely gone, skip as before; otherwise proceed with the fresh object and write the status.

The extra uncached read happens only on a cache miss — never for stable, already-cached objects — so it is bounded by create/delete churn rather than the status-event rate and adds no meaningful API-server load in steady state.

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.60%. Comparing base (bae64fd) to head (77ea943).

Files with missing lines	Patch %	Lines
internal/provider/kubernetes/status_updater.go	62.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9566   +/-   ##
=======================================
  Coverage   75.59%   75.60%           
=======================================
  Files         252      252           
  Lines       41747    41751    +4     
=======================================
+ Hits        31559    31564    +5     
+ Misses       8061     8060    -1     
  Partials     2127     2127           

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