refactor: update PolicyObservedClusterCount in one place in collectScheduledClusters

[Copilot]

PolicyObservedClusterCount was written in two places: determinePolicySnapshot set it to -1 as a sentinel for PickAll policy, then collectScheduledClusters detected that sentinel and overwrote it with len(selectedBindings).

Changes

• determinePolicySnapshot: Remove the PolicyObservedClusterCount assignment entirely; update the klog to reference the local clusterCount variable directly.
• collectScheduledClusters: Add clusterCount int parameter. Validate count mismatch first, then set PolicyObservedClusterCount = len(selectedBindings) unconditionally in one place — valid for both PickAll (clusterCount == -1) and other policies (where the preceding check guarantees clusterCount == len(selectedBindings)).
• Callers: initialize now captures and forwards clusterCount; validate forwards its existing clusterCount.

// Before: two assignments, sentinel-based indirection
updateRunStatus.PolicyObservedClusterCount = clusterCount // -1 for PickAll, set in determinePolicySnapshot
...
if updateRunStatus.PolicyObservedClusterCount == -1 {     // overwritten in collectScheduledClusters
    updateRunStatus.PolicyObservedClusterCount = len(selectedBindings)
} else if updateRunStatus.PolicyObservedClusterCount != len(selectedBindings) { ... }

// After: single assignment in collectScheduledClusters
if clusterCount != -1 && clusterCount != len(selectedBindings) {
    return nil, nil, fmt.Errorf(...)  // mismatch error
}
updateRunStatus.PolicyObservedClusterCount = len(selectedBindings)  // always set here