Optional dedicated :class:~langsmith.Client for this replica.
When set, the replica's runs are enqueued on this client's tracing queue
(and dispatched by its background thread) instead of the RunTree's default
client. This lets each replica use a different tracing mode — for example,
one replica with Client(tracing_mode="otel") and another with the
default LangSmith-only client.
The field is not propagated in distributed-tracing baggage (each service
must construct its own clients).
