test(langchain): mark legacy trigger view for 2.0 removal (#38002)

`SummarizationMiddleware._trigger_conditions` is now explicitly marked
as a temporary compatibility view for private consumers. The regression
test is tied to the package major version so the 2.0 release path fails
loudly until the legacy attr and test are removed.

libs/langchain_v1/langchain/agents/middleware/summarization.py

@@ -327,7 +327,7 @@ class SummarizationMiddleware(AgentMiddleware[AgentState[ResponseT], ContextT, R
         self._trigger_clauses = self._normalize_trigger(self.trigger)
         # Legacy compatibility view for private consumers that inspected the previous
         # tuple-normalized representation. LangChain behavior is driven by
-        # `_trigger_clauses`, not this attribute.
+        # `_trigger_clauses`, not this attribute. Remove in LangChain 2.0.
         self._trigger_conditions = self._legacy_trigger_conditions(self.trigger)
 
         self.keep = self._validate_context_size(keep, "keep")

libs/langchain_v1/tests/unit_tests/agents/middleware/implementations/test_summarization.py

@@ -1,4 +1,5 @@
 from collections.abc import Iterable
+from pathlib import Path
 from typing import Any
 from unittest.mock import patch
 
@@ -36,6 +37,20 @@ from langchain.chat_models import init_chat_model
 from tests.unit_tests.agents.model import FakeToolCallingModel
 
 
+def _langchain_pyproject_major_version() -> int:
+    """Read the `langchain` package major version from `pyproject.toml`."""
+    pyproject = next(
+        parent / "pyproject.toml"
+        for parent in Path(__file__).parents
+        if (parent / "pyproject.toml").exists()
+    )
+    for line in pyproject.read_text().splitlines():
+        if line.startswith("version = "):
+            return int(line.split('"')[1].split(".")[0])
+    msg = "Could not find project version in pyproject.toml"
+    raise AssertionError(msg)
+
+
 class MockChatModel(BaseChatModel):
     """Mock chat model for testing."""
 
@@ -1173,8 +1188,11 @@ def test_trigger_clauses_are_canonical_representation() -> None:
     ]
 
 
-def test_trigger_conditions_preserve_legacy_tuple_view() -> None:
-    """Test `_trigger_conditions` remains a tuple-shaped compatibility view."""
+def test_trigger_conditions_legacy_tuple_view_remove_in_2_0() -> None:
+    """Test `_trigger_conditions` remains a temporary tuple-shaped compatibility view."""
+    assert _langchain_pyproject_major_version() < 2, (
+        "Remove `_trigger_conditions` and this compatibility test in LangChain 2.0."
+    )
     middleware = SummarizationMiddleware(
         model=FakeToolCallingModel(),
         trigger=[("messages", 5), {"tokens": 1000}, {"tokens": 2000, "messages": 10}],