test: Add failing test for BaseCallbackManager.merge (#32040)

This pull request introduces a failing unit test to reproduce the bug
reported in issue #32028.
The test asserts the expected behavior: `BaseCallbackManager.merge()`
should combine `handlers` and `inheritable_handlers` independently,
without mixing them. This test will fail on the current codebase and is
intended to guide the fix and prevent future regressions.

---------

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>
Co-authored-by: Mason Daugherty <mason@langchain.dev>

libs/core/tests/unit_tests/callbacks/test_sync_callback_manager.py

@@ -1,3 +1,5 @@
+import pytest
+
 from langchain_core.callbacks.base import BaseCallbackHandler, BaseCallbackManager
 
 
@@ -13,3 +15,29 @@ def test_remove_handler() -> None:
     manager = BaseCallbackManager([handler1], inheritable_handlers=[handler2])
     manager.remove_handler(handler1)
     manager.remove_handler(handler2)
+
+
+@pytest.mark.xfail(
+    reason="TODO: #32028 merge() incorrectly mixes handlers and inheritable_handlers"
+)
+def test_merge_preserves_handler_distinction() -> None:
+    """Test that merging managers preserves the distinction between handlers.
+
+    This test verifies the correct behavior of the BaseCallbackManager.merge()
+    method. When two managers are merged, their handlers and
+    inheritable_handlers should be combined independently.
+
+    Currently, it is expected to xfail until the issue is resolved.
+    """
+    h1 = BaseCallbackHandler()
+    h2 = BaseCallbackHandler()
+    ih1 = BaseCallbackHandler()
+    ih2 = BaseCallbackHandler()
+
+    m1 = BaseCallbackManager(handlers=[h1], inheritable_handlers=[ih1])
+    m2 = BaseCallbackManager(handlers=[h2], inheritable_handlers=[ih2])
+
+    merged = m1.merge(m2)
+
+    assert set(merged.handlers) == {h1, h2}
+    assert set(merged.inheritable_handlers) == {ih1, ih2}