docs: clarify rpc client mixin contract

src/wf_transport_rpc_http/client.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from dataclasses import dataclass
+
 import httpx  # noqa: F401  # Backcompat for tests patching client.httpx.AsyncClient.
 
 from .client_artifacts import RpcArtifactClientMixin
@@ -10,6 +12,7 @@ from .client_drafts import RpcDraftClientMixin
 from .client_runs import RpcRunClientMixin
 
 
+@dataclass(slots=True)
 class RpcWorkflowApiClient(
     RpcClientTransport,
     RpcCapabilityClientMixin,
@@ -18,7 +21,13 @@ class RpcWorkflowApiClient(
     RpcDeploymentClientMixin,
     RpcRunClientMixin,
 ):
-    """WorkflowApiSurface implementation backed by JSON-RPC HTTP calls."""
+    """WorkflowApiSurface implementation backed by JSON-RPC HTTP calls.
+
+    The inheritance order is intentional: `RpcClientTransport` owns dataclass
+    fields and `_call`; domain mixins are stateless method groups that may only
+    depend on `_call`. If a domain needs state or lifecycle later, prefer a
+    composed domain client instead of adding fields to a mixin.
+    """
 
 
 __all__ = ["RpcWorkflowApiClient"]