test(streaming): pin that reasoning_items are concatenated across chunks

The reasoning_items fix landed upstream. Its test puts every item in the
last chunk, so an aggregation that read only the first or last chunk would
still pass. Add one case with an item in each of two chunks and expect both,
in order.

tests/unit/litellm_core_utils/test_streaming_chunk_builder_utils.py

@@ -2517,3 +2517,44 @@ def test_stream_chunk_builder_keeps_reasoning_items_on_text_only_replies():
     assert response is not None
     assert response.choices[0].message.content == "Hello world"
     assert response.choices[0].message.reasoning_items == reasoning_items
+
+
+def test_stream_chunk_builder_concatenates_reasoning_items_from_every_chunk():
+    from litellm.types.utils import Delta, ModelResponseStream, StreamingChoices
+
+    first: Final = {"type": "reasoning", "id": "rs_1", "summary": [], "encrypted_content": "enc_1"}
+    second: Final = {"type": "reasoning", "id": "rs_2", "summary": [], "encrypted_content": "enc_2"}
+    chunks: Final = [
+        ModelResponseStream(
+            id="chatcmpl-rs",
+            created=1,
+            model="gpt-5.5",
+            object="chat.completion.chunk",
+            choices=[
+                StreamingChoices(
+                    finish_reason=None,
+                    index=0,
+                    delta=Delta(content="Hello", role="assistant", reasoning_items=[first]),
+                )
+            ],
+        ),
+        ModelResponseStream(
+            id="chatcmpl-rs",
+            created=1,
+            model="gpt-5.5",
+            object="chat.completion.chunk",
+            choices=[
+                StreamingChoices(
+                    finish_reason="stop",
+                    index=0,
+                    delta=Delta(content=" world", role="assistant", reasoning_items=[second]),
+                )
+            ],
+        ),
+    ]
+
+    response: Final = stream_chunk_builder(chunks=list(chunks))
+
+    assert response is not None
+    assert response.choices[0].message.content == "Hello world"
+    assert response.choices[0].message.reasoning_items == [first, second]