gce: dynamically refresh GCE managed zones on node informer events

[arvindbr8]

What this PR does / why we need it

This PR implements a dynamic zone refresh mechanism for the GCE cloud provider in the Cloud Controller Manager (CCM).
Currently, CCM caches the GCE zones it manages (managedZones) exactly once at startup and never refreshes them. When a new zone is enabled in the GCP region (e.g., when adding a node pool in a new zone), all zonal VM/node lookups (getInstanceByName) and persistent volume operations (GetDiskByNameUnknownZone) fail in the new zone because the cache is stale. Previously, this required a manual Kubernetes Control Plane (KCP) restart to get CCM to recognize the new zone.

How it solves it

We introduce a zero-downtime, thread-safe, on-demand zone refresh:

1. Thread Safety: Added sync.RWMutex (managedZonesLock) to the GCE Cloud struct and refactored all read occurrences to use a thread-safe getter (getManagedZones()).
2. Mock-Friendly API: Refactored the zone listing refresh to use the modern ListZonesInRegion interface instead of the legacy raw getZonesForRegion API, allowing standard GCE mock testing.
3. Event-Driven Trigger: Hooked the refresh into the node informer (updateNodeZones). When a node registers in a zone that is missing from CCM's cache, CCM dynamically triggers refreshManagedZones() to fetch the expanded zone list.
4. Deadlock Prevention: Structurally optimized updateNodeZones to compute local state and manually release the nodeZonesLock before triggering any GCE API calls.

Special notes for your reviewer

• The node informer was chosen as the trigger because GKE volume provisioning requires a node to exist in a zone before volumes can be targeted there. This event-driven hook provides zero idle GCE API overhead.
• I added a complete unit test TestUpdateNodeZonesDynamicRefresh in gce_instances_test.go which simulates the node informer events pipeline and verifies the dynamic cache refresh.**

cc: @YifeiZhuang @zhaoqsh @gnossen

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[YifeiZhuang]

/ok-to-test