fix(channels): copy configurable dict before injecting run-scoped fields

When configurable was already a plain dict, _resolve_run_params mutated
  it in place, leaking checkpoint_ns and thread_id back into the shared
  session config. Always copy via dict() before mutating to prevent
  cross-user or cross-channel config pollution.

backend/app/channels/manager.py

@@ -590,9 +590,7 @@ class ChannelManager:
         )
 
         configurable = run_config.get("configurable")
-        if isinstance(configurable, dict):
-            pass
-        elif isinstance(configurable, Mapping):
+        if isinstance(configurable, Mapping):
             configurable = dict(configurable)
         else:
             configurable = {}