fix(control-plane): drop -z- from MPP namespace prefix (#1178)

## Summary

- Fixes hardcoded `ambient-code--z-` prefix in `MPPNamespaceProvisioner`
— the tenant-operator materializes `TenantNamespace` CRs as
`ambient-code--<id>`, not `ambient-code--z-<id>`
- With the wrong prefix, `ProvisionNamespace` would create the
`TenantNamespace` CR correctly but then poll for `ambient-code--z-<id>`
which never appeared, timing out after 60s every time

## Test plan
- [ ] Create a session on MPP cluster — namespace provisioned as
`ambient-code--<project-id>` and becomes active without timeout

🤖 Generated with [Claude Code](https://claude.ai/code)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Updated namespace naming conventions in the control plane provisioning
system.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: markturansky

components/ambient-control-plane/internal/kubeclient/namespace_provisioner.go

@@ -97,7 +97,7 @@ func NewMPPNamespaceProvisioner(kube *KubeClient, configNamespace string, logger
 	}
 }
 
-const mppNamespacePrefix = "ambient-code--z-"
+const mppNamespacePrefix = "ambient-code--"
 
 func (p *MPPNamespaceProvisioner) instanceID(namespaceName string) string {
 	if len(namespaceName) > len(mppNamespacePrefix) && namespaceName[:len(mppNamespacePrefix)] == mppNamespacePrefix {