feat: add ListRunnerScaleSets on Client

[niranjan94]

Summary

Adds Client.ListRunnerScaleSets(ctx, runnerGroupID) which returns every runner scale set in a given runner group.

Today the Client exposes GetRunnerScaleSet (lookup by name within a group) and GetRunnerScaleSetByID (lookup by ID), but there is no way to enumerate scale sets in a group without already knowing their names or IDs. This makes it awkward for external controllers that own a subset of scale sets in a group and need to reconcile their desired state against what currently exists on the service, for example to detect and clean up scale sets whose owning configuration has been removed.

The new method hits the same runnerscalesets endpoint that GetRunnerScaleSet already uses, just without the name= query filter, and decodes the existing runnerScaleSetsResponse envelope. No new types, no change to existing behavior.

Changes

• client.go: new ListRunnerScaleSets method, modeled on the style and locking discipline of the neighboring GetRunnerScaleSet / GetRunnerScaleSetByID methods.
• client_test.go: TestListRunnerScaleSets verifies the request path, the runnerGroupId query parameter, the absence of a name filter, and that the response is decoded into the returned slice.