fix(core): treat status as a last-wins field when merging chunks

"status" fields on streamed content blocks (e.g. a Responses API
function_call moving from "in_progress" to "completed") are enum-like
state, not accumulated text. merge_dicts previously concatenated two
string "status" values like any other string, corrupting them into
values like "in_progresscompleted" once a provider integration started
carrying status across chunks.

Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>

libs/core/langchain_core/utils/_merge.py

@@ -61,6 +61,11 @@ def merge_dicts(left: dict[str, Any], *others: dict[str, Any]) -> dict[str, Any]
                     and merged[right_k] == right_v
                 ):
                     continue
+                if right_k == "status":
+                    # Enum-like state (e.g. "in_progress" -> "completed") that a
+                    # later chunk overwrites, not accumulated text.
+                    merged[right_k] = right_v
+                    continue
                 merged[right_k] += right_v
             elif isinstance(merged[right_k], dict):
                 merged[right_k] = merge_dicts(merged[right_k], right_v)

libs/core/tests/unit_tests/utils/test_utils.py

@@ -129,6 +129,11 @@ def test_check_package_version(
         # Other integer fields should still be summed (e.g., token counts)
         ({"tokens": 10}, {"tokens": 5}, {"tokens": 15}),
         ({"count": 1}, {"count": 2}, {"count": 3}),
+        # 'status' is enum-like state that a later chunk overwrites, not
+        # accumulated text (e.g. streamed Responses API items going from
+        # "in_progress" to "completed").
+        ({"status": "in_progress"}, {"status": "completed"}, {"status": "completed"}),
+        ({"status": "completed"}, {"status": "completed"}, {"status": "completed"}),
     ],
 )
 def test_merge_dicts(