test(core): add regression test for list-index $ref resolution (#34097)

This PR adds a regression test covering the JSON Schema `$ref` pattern
found in
MCP-style schemas, where a `$ref` points into a list-based structure
such as:


#/properties/body/anyOf/1/properties/Message/properties/bccRecipients/items

This pattern historically failed due to incorrect handling of numeric
list
components in `_retrieve_ref`. The underlying bug has since been fixed,
and
this test ensures coverage so we don't regress on list-index `$ref`
resolution.

The new test (`test_dereference_refs_list_index_items_ref_mcp_like`)
verifies:

- correct traversal into `anyOf[1]`
- proper dereferencing of `items.$ref`
- no errors thrown
- `ccRecipients.items` is identical to the resolved schema of
`bccRecipients.items`

No code changes are included, just the one test — this PR adds coverage
to preserve the expected
behavior and documents support for this real-world MCP schema pattern.

Related to #32012.

---------

Co-authored-by: Mason Daugherty <mason@langchain.dev>

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

@@ -452,6 +452,84 @@ def test_dereference_refs_list_index() -> None:
     assert actual_dict_key == expected_dict_key
 
 
+def test_dereference_refs_list_index_items_ref_mcp_like() -> None:
+    """Regression test: MCP-style list index ref into array items."""
+    schema = {
+        "type": "object",
+        "properties": {
+            "body": {
+                "anyOf": [
+                    {"type": "string"},
+                    {
+                        "type": "object",
+                        "properties": {
+                            "Message": {
+                                "type": "object",
+                                "properties": {
+                                    "bccRecipients": {
+                                        "type": "array",
+                                        "items": {
+                                            "type": "object",
+                                            "properties": {
+                                                "emailAddress": {
+                                                    "type": "object",
+                                                    "properties": {
+                                                        "address": {"type": "string"},
+                                                        "name": {"type": "string"},
+                                                    },
+                                                    "required": ["address"],
+                                                }
+                                            },
+                                        },
+                                        "description": (
+                                            "The Bcc: recipients for the message."
+                                        ),
+                                    },
+                                    "ccRecipients": {
+                                        "type": "array",
+                                        "items": {
+                                            "$ref": (
+                                                "#/properties/body/anyOf/1/"
+                                                "properties/Message/properties/"
+                                                "bccRecipients/items"
+                                            )
+                                        },
+                                        "description": (
+                                            "The Cc: recipients for the message."
+                                        ),
+                                    },
+                                },
+                                "additionalProperties": False,
+                            },
+                            "SaveToSentItems": {
+                                "type": ["boolean", "null"],
+                                "default": False,
+                            },
+                        },
+                        "additionalProperties": False,
+                    },
+                ]
+            }
+        },
+        "required": ["body"],
+        "additionalProperties": False,
+    }
+
+    resolved = dereference_refs(schema)
+
+    message_props = resolved["properties"]["body"]["anyOf"][1]["properties"]["Message"][
+        "properties"
+    ]
+
+    bcc_items = message_props["bccRecipients"]["items"]
+    cc_items = message_props["ccRecipients"]["items"]
+
+    # $ref should be fully resolved in ccRecipients.items
+    assert "$ref" not in cc_items
+    # And ccRecipients.items should match bccRecipients.items
+    assert cc_items == bcc_items
+
+
 def test_dereference_refs_mixed_ref_with_properties() -> None:
     """Test dereferencing refs that have $ref plus other properties."""
     # This pattern can cause infinite recursion if not handled correctly