community: add truncation params when an openai assistant's run is created (#28158)

**Description:** When an OpenAI assistant is invoked, it creates a run
by default, allowing users to set only a few request fields. The
truncation strategy is set to auto, which includes previous messages in
the thread along with the current question until the context length is
reached. This causes token usage to grow incrementally:
consumed_tokens = previous_consumed_tokens + current_consumed_tokens.

This PR adds support for user-defined truncation strategies, giving
better control over token consumption.

**Issue:** High token consumption.

libs/community/langchain_community/agents/openai_assistant/base.py

@@ -543,11 +543,16 @@ class OpenAIAssistantV2Runnable(OpenAIAssistantRunnable):
         Returns:
             Any: The created run object.
         """
-        params = {
-            k: v
-            for k, v in input.items()
-            if k in ("instructions", "model", "tools", "tool_resources", "run_metadata")
-        }
+        allowed_assistant_params = (
+            "instructions",
+            "model",
+            "tools",
+            "tool_resources",
+            "run_metadata",
+            "truncation_strategy",
+            "max_prompt_tokens",
+        )
+        params = {k: v for k, v in input.items() if k in allowed_assistant_params}
         return self.client.beta.threads.runs.create(
             input["thread_id"],
             assistant_id=self.assistant_id,

libs/community/tests/unit_tests/agents/test_openai_assistant.py

@@ -0,0 +1,39 @@
+from typing import Any
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from langchain_community.agents.openai_assistant import OpenAIAssistantV2Runnable
+
+
+def _create_mock_client(*args: Any, use_async: bool = False, **kwargs: Any) -> Any:
+    client = AsyncMock() if use_async else MagicMock()
+    client.beta.threads.runs.create = MagicMock(return_value=None)  # type: ignore
+    return client
+
+
+@pytest.mark.requires("openai")
+def test_set_run_truncation_params() -> None:
+    client = _create_mock_client()
+
+    assistant = OpenAIAssistantV2Runnable(assistant_id="assistant_xyz", client=client)
+    input = {
+        "content": "AI question",
+        "thread_id": "thread_xyz",
+        "instructions": "You're a helpful assistant; answer questions as best you can.",
+        "model": "gpt-4o",
+        "max_prompt_tokens": 2000,
+        "truncation_strategy": {"type": "last_messages", "last_messages": 10},
+    }
+    expected_response = {
+        "assistant_id": "assistant_xyz",
+        "instructions": "You're a helpful assistant; answer questions as best you can.",
+        "model": "gpt-4o",
+        "max_prompt_tokens": 2000,
+        "truncation_strategy": {"type": "last_messages", "last_messages": 10},
+    }
+
+    assistant._create_run(input=input)
+    _, kwargs = client.beta.threads.runs.create.call_args
+
+    assert kwargs == expected_response