[HorizonDB] Private endpoint connection update should use PUT, not PATCH

Follow-up to Azure/azure-sdk-for-python#47688 (which reported the missing cluster name in the private endpoint connection URL). The **route is now correctly cluster-scoped**, but the **update operation is still generated as `PATCH`**, while the HorizonDB backend private endpoint connection (PEC) controller and the Azure CLI extension use **`PUT`** for approve/reject.

### Current behavior

Generated OpenAPI for `2026-01-20-preview` emits `patch` under the cluster-scoped path:

```
/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.HorizonDb/clusters/{clusterName}/privateEndpointConnections/{privateEndpointConnectionName}
```

with operationId `HorizonDbPrivateEndpointConnections_Update`.

### Expected behavior

The same path / operationId should emit `put`.

### Precedent

The sibling service **PostgreSQL Flexible Server** models this operation as `PUT` (full `PrivateEndpointConnection` body, `202`-only response, `final-state-via: azure-async-operation`) in both `stable/2025-08-01` and `preview/2026-04-01-preview`:

- `specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/stable/2025-08-01/openapi.json`

### Root cause

`specification/horizondb/resource-manager/Microsoft.HorizonDb/HorizonDb/routes.tsp` uses a PATCH template:

```typespec
update is HorizonDbPrivateEndpointOperations.CustomPatchAsync<
HorizonDbCluster,
PrivateEndpointConnectionResource
>;
```

It should use a PUT-capable template (e.g. `CreateOrUpdateAsync`) so the generated verb is `PUT`.

### Impact

SDK regeneration reverts downstream consumers back to PATCH until the spec is fixed. The Azure CLI extension (`azure-cli-extensions/src/horizondb`) currently hand-patches the vendored Python SDK to force `method="PUT"`; that workaround can be removed once the spec emits PUT and the SDK is regenerated.

Start in specification/horizondb/resource-manager/Microsoft.HorizonDb/HorizonDb/routes.tsp at the cluster-scoped private endpoint connection update definition, then compare the PostgreSQL Flexible Server OpenAPI precedent. Replace the PATCH-capable template with the PUT-capable one and verify that the generated 2026-01-20-preview operation keeps the same path and operationId while emitting PUT.

Written by the indexing model from the issue text.