Send temporal-namespace header from ActivityClient

[Sebruck]

What was changed

ActivityClientImpl now wraps its WorkflowServiceStubs in NamespaceInjectWorkflowServiceStubs, like WorkflowClientInternalImpl, ScheduleClientImpl and NexusClientImpl already do.

Why?

Without the temporal-namespace header, Standalone Activity calls (StartActivityExecution and the rest of the ActivityClient API) are rejected by Temporal Cloud for API-key authentication with PERMISSION_DENIED: Request unauthorized. Workflow starts through the same stubs and key work. We hit this in production use against Temporal Cloud and currently work around it by wrapping the stubs ourselves.

How was this tested

Added AuthorizationTokenTest.activityClientRequestsShouldHaveANamespace, which records the outgoing headers of StartActivityExecution. It fails without the change (expected:<default> but was:<null>) and passes with it.

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