fix(langgraph): merge configurable dicts across configs in ensure_config

Previously, when ensure_config was called with multiple configs (e.g.
Pregel.stream's ensure_config(self.config, config)), a later config's
configurable dict fully overwrote an earlier one. This caused values
bound via with_config({"configurable": {...}}) to be silently dropped
whenever the invoke-time config supplied any other configurable key.

Concretely, create_agent binds {configurable: {ls_agent_type: 'root'}}
via with_config, and any real invocation with a checkpointer supplies
{configurable: {thread_id: ...}} at invoke time. The bound ls_agent_type
was dropped, so no root-level runs were tagged with ls_agent_type='root'
in LangSmith.

Fix: merge the configurable dict across configs (stdlib merge_configs in
langchain_core already does this correctly; langgraph's merge_configs
helper in this same file also does this at line 109). Invoke-time values
still override bound values when keys collide.

Adds a regression test in tests/test_utils.py.

libs/langgraph/tests/test_utils.py

@@ -427,3 +427,32 @@ def test_callback_manager_copies_configurable_ids_to_tracing_metadata() -> None:
         "thread_id": "th-123",
         "user_id": "uid-1",
     }
+
+
+def test_ensure_config_merges_configurable_across_configs() -> None:
+    """`ensure_config(bound, invoke_time)` should merge `configurable` dicts.
+
+    Prior to the fix, a later config's `configurable` dict fully overwrote an
+    earlier one, causing values bound via `with_config({"configurable": {...}})`
+    (e.g. `ls_agent_type="root"` set by `create_agent`) to be dropped whenever
+    an invoke-time config supplied any other configurable key like `thread_id`.
+    """
+    bound: RunnableConfig = {
+        "configurable": {"ls_agent_type": "root", "custom_setting": "keep_me"},
+        "metadata": {"ls_integration": "langchain_create_agent"},
+    }
+    invoke_time: RunnableConfig = {
+        "configurable": {"thread_id": "t-1"},
+    }
+    merged = ensure_config(bound, invoke_time)
+    # Both the bound and invoke-time configurable keys are preserved.
+    assert merged["configurable"] == {
+        "ls_agent_type": "root",
+        "custom_setting": "keep_me",
+        "thread_id": "t-1",
+    }
+    # Invoke-time values still override bound values when they collide.
+    override: RunnableConfig = {"configurable": {"ls_agent_type": "subagent"}}
+    merged2 = ensure_config(bound, override)
+    assert merged2["configurable"]["ls_agent_type"] == "subagent"
+    assert merged2["configurable"]["custom_setting"] == "keep_me"