[translation]: Answer ListNamespaces from CloudService.GetNamespaces

[Vaughan-Temporal]

Stacked on #152. Review that first; this diff is only the mapping.

WorkflowService.ListNamespaces cannot be called on Temporal Cloud on any endpoint. Cloud pins a request's namespace to
the endpoint hostname and rejects any request whose body namespace differs; ListNamespacesRequest has no namespace
field, so its body namespace is always empty and the check can never pass. Cloud exposes the same capability as
CloudService.GetNamespaces on saas-api — a different service, different message types, different host.

Registers that translation using the library from #152.

The judgement calls

All the lossy decisions live in namespaces.go and are the thing to review:

• State. Cloud's eleven ResourceState values fold onto Temporal's three. Computed once so the deleted filter and
  the reported state cannot disagree.
• Retention. Cloud reports whole days; config is left unset when it reports none, rather than claiming a zero
  retention Cloud never stated.
• ReplicationConfig deliberately unset. Cloud describes replication as regional replicas, not Temporal clusters.
  Filling it with region ids would have clients read them as cluster names. IsGlobalNamespace is derived from the
  replica count instead.
• include_deleted has nowhere to go on the Cloud request, so it is applied to the response.
• No namespace UUID, description or owner email — Cloud has no equivalent, so those stay empty.

Also

temporal-cloud-api-version is required (GetNamespaces fails InvalidArgument without it) and pinned to
cloudclient.DefaultAPIVersion(), so it moves with the module the message types come from.

CloudService is added to the linked service descriptors so the method resolves.

Scope

+482 / −8. Nothing installs the registry yet, so no request is translated. That is #3 in the stack.

⚠️ Not yet verified against a live Cloud account. v0.20.0 for the API version is the most likely thing to be wrong on
first contact.

---

Stack

	PR
1	#152	translation library — mechanism only
2	#153	ListNamespaces → CloudService.GetNamespaces mapping
3	#154	integration — first behaviour change
4	#155	listnamespace example

Merge bottom-up. Each PR targets the one above it, so its diff shows only its own change.

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