feat(azure-network): Public IP Prefixes

[NitinKumar004]

What

Adds Microsoft.Network/publicIPPrefixes (CRUD + list) to the Azure vnet wire handler, driven end-to-end by the real armnetwork.PublicIPPrefixesClient.

• On create the provider synthesizes properties.ipPrefix — a CIDR of the requested prefixLength carved from a deterministic 10.0.0.0/8 allocator (monotonic /24 block counter, no randomness). prefixLength defaults to 28 when omitted and is immutable on re-PUT.
• sku is top-level (Standard/StandardV2, tier Regional), echoed back on GET.
• Read-only publicIPAddresses[] back-reference: rebuilt by scanning public IPs whose publicIPPrefix ref matches this prefix. A public IP created with a publicIPPrefix ref only stores the ref (child-IP allocation from the prefix range is deferred); the public IP echoes the prefix on its own GET.

Architecture fit

• LRO: PUT + DELETE ride the vnet handler's existing async plumbing (writeAcceptedAsync + the locations/operationStatuses responder) — the same pattern as NAT gateways, so BeginCreateOrUpdate/BeginDelete pollers terminate cleanly.
• Snapshot: new store bolts onto the already-Snapshottable vnet Mock; since the completeness guard only checks the field type, the store was manually added to the provider's snapshot dumps/loads list and a round-trip assertion added to snapshot_test.go.
• Purge: added to the PurgeResourceGroup cascade so prefixes don't orphan on RG delete.
• ARG triple (Azure discovery gaps block cost E2E: Container Apps, Azure ML, Storage Account ARG projection #334): type-asserted walker in resourcediscovery, portable TypePublicIPPrefix const, and resourcegraph forward/reverse map entries. All additions are Azure-gated — AWS/GCP ARNs and Resource Graph rows are byte-unchanged (the walker fails the type assertion on AWS/GCP; the arn.go entry only extends the Azure type map).
• Azure-only surface modeled as an optional, type-asserted AzurePublicIPPrefixes capability on the networking driver (mirrors AzureApplicationSecurityGroups); the cross-cloud driver is not widened.
• docs/coverage/ regenerated.

Test

Real-SDK e2e through the pollers (server/azure/vnet/public_ip_prefix_test.go):

• BeginCreateOrUpdate (prefixLength=28) → PollUntilDone → Get round-trips a synthesized ipPrefix of the right size + top-level sku; list; BeginDelete → subsequent Get returns 404.
• Back-reference test: a public IP created with a publicIPPrefix ref appears in the prefix's read-only publicIPAddresses[] and echoes the prefix.
• Provider snapshot/restore round-trip asserts the synthesized CIDR + sku survive.

Refs #611.