fix(multi-slb): fix race condition for backendpoolUpdater

[hebo4096]

What type of PR is this?

/kind bug

What this PR does / why we need it:

In multi-SLB clusters with ExternalTrafficPolicy: Local the loadBalancerBackendPoolUpdater.process() and the main service reconciliation loop (i.e., reconcileLoadBalancer → CreateOrUpdateLB) can issue concurrent ARM calls targeting the same load balancer.
Because these are parent PUT (LB-level) and child PUT (backend pool-level) operations.
ARM rejects the operation with HTTP 412 (ETag mismatch / PreconditionFailed) when they race.

This PR fixes the race by splitting process() into two phases:

Dequeue — drains the operation queue under the existing queue lock (updater.lock), then releases it.
ARM operations — performs Get/CreateOrUpdate under serviceReconcileLock, serializing with the main reconciliation loop.
This ensures that backend pool PUTs and LB PUTs never overlap, eliminating the ETag conflict. The queue lock is released before acquiring serviceReconcileLock to avoid nested locks and to keep addOperation() non-blocking during ARM calls.

Which issue(s) this PR fixes:

Fixes #9839

Special notes for your reviewer:

The core fix is in azure_local_services.go: process() is split into dequeue(), and ARM operations under serviceReconcileLock.

Two new unit tests verify the lock behavior:
TestLoadBalancerBackendPoolUpdaterSerialization - ARM calls are blocked while serviceReconcileLock is held.
TestLoadBalancerBackendPoolUpdaterDequeueUnderQueueLock - queue lock is released before ARM calls, so addOperation() for backendPoolUpdater is not blocked.

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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• ✅ login: hebo4096 / name: Shogo Shimahara (b86b388)

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