fix: don't create empty-zone offerings for zone-restricted SKUs

[k24dizzle]

Problem

Fixes #1384

When using TopologySpreadConstraints with whenUnsatisfiable: DoNotSchedule, Karpenter refuses to provision new nodes even when valid zones are available.

The root cause is that some VM SKUs have NotAvailableForSubscription zone restrictions covering all zones in a region. skewer correctly strips those blocked zones, leaving an empty zone set. instanceTypeZones() then falls through to sets.New("") — a fallback originally intended for truly non-zonal regions — causing these SKUs to appear as available with zone="".

Karpenter core treats "" as a valid topology domain, so instead of seeing zone counts [2, 1, 1] across 3 real zones, the scheduler sees [0, 2, 1, 1] across 4 domains. With maxSkew: 1, this makes the constraint unsatisfiable even though valid zones exist.

See kubernetes-sigs/karpenter#2864 for an attempt to fix this in the upstream karpenter, this PR is an attempt to fix this here in the azure provider by not offering these instances in the first place.

Fix

Add hasRawZonesInRegion() to check locationInfo directly (before restriction filtering). If a SKU has zones defined in the region but none are available after restrictions are applied, return an empty set instead of sets.New(""). This causes the SKU to produce no offerings and be dropped from the instance type list. The sets.New("") path for truly non-zonal regions is preserved.

Verification

In affected subscriptions, SKUs like Standard_L48s_v3 report zone support in locationInfo but carry NotAvailableForSubscription restrictions covering all zones. After the fix, instance type count in westus2 drops from 507 → 496, zone="" offerings drop to 0, and TSC scheduling with whenUnsatisfiable: DoNotSchedule works correctly.

Inspecting a zone-restricted SKU with az vm list-skus

az vm list-skus \
  --location westus2 \
  --resource-type virtualMachines \
  --query "[?name=='Standard_L48s_v3'].{name:name,locationInfo:locationInfo,restrictions:restrictions}" \
  --output json

{
  "locationInfo": [{ "zones": ["1", "2", "3"] }],
  "restrictions": [{
    "reasonCode": "NotAvailableForSubscription",
    "type": "Zone",
    "restrictionInfo": { "zones": ["1", "2", "3"] }
  }]
}

[k24dizzle]

@microsoft-github-policy-service agree

[tallaxes]

Closing in favor of #1615