use proto field path for Cloud Run scale update mask

[gcristea-temporal]

What was changed

Use the proto field path scaling.manual_instance_count and build the mask via fieldmaskpb.New so an invalid path panics at load instead of misbehaving silently at runtime.

Why?

UpdateWorkerSetSize sent the scale update mask as scaling.manualInstanceCount (camelCase JSON name).

Cloud Run silently ignores an unknown mask path and applies the sparse WorkerPool wholesale, resetting the pool's runtime serviceAccount to the project default compute SA — which surfaces as an iam.serviceAccounts.actAs PermissionDenied on the deployer.

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

[02strich]

Is this a real problem given it is a constant we are dealing with?

[02strich]

After chatting with the author, it is a real thing b/c Google docs are buggy and so figuring out the right constant is unreliable