langchain[patch]: deprecate various chains (#25310)

- [x] NatbotChain: move to community, deprecate langchain version.
Update to use `prompt | llm | output_parser` instead of LLMChain.
- [x] LLMMathChain: deprecate + add langgraph replacement example to API
ref
- [x] HypotheticalDocumentEmbedder (retriever): update to use `prompt |
llm | output_parser` instead of LLMChain
- [x] FlareChain: update to use `prompt | llm | output_parser` instead
of LLMChain
- [x] ConstitutionalChain: deprecate + add langgraph replacement example
to API ref
- [x] LLMChainExtractor (document compressor): update to use `prompt |
llm | output_parser` instead of LLMChain
- [x] LLMChainFilter (document compressor): update to use `prompt | llm
| output_parser` instead of LLMChain
- [x] RePhraseQueryRetriever (retriever): update to use `prompt | llm |
output_parser` instead of LLMChain

libs/community/tests/unit_tests/chains/test_natbot.py

@@ -0,0 +1,58 @@
+"""Test functionality related to natbot."""
+
+from typing import Any, Dict, List, Optional
+
+from langchain.chains.natbot.base import NatBotChain
+from langchain_core.callbacks.manager import CallbackManagerForLLMRun
+from langchain_core.language_models.llms import LLM
+
+
+class FakeLLM(LLM):
+    """Fake LLM wrapper for testing purposes."""
+
+    def _call(
+        self,
+        prompt: str,
+        stop: Optional[List[str]] = None,
+        run_manager: Optional[CallbackManagerForLLMRun] = None,
+        **kwargs: Any,
+    ) -> str:
+        """Return `foo` if longer than 10000 words, else `bar`."""
+        if len(prompt) > 10000:
+            return "foo"
+        else:
+            return "bar"
+
+    @property
+    def _llm_type(self) -> str:
+        """Return type of llm."""
+        return "fake"
+
+    def get_num_tokens(self, text: str) -> int:
+        return len(text.split())
+
+    @property
+    def _identifying_params(self) -> Dict[str, Any]:
+        return {}
+
+
+def test_proper_inputs() -> None:
+    """Test that natbot shortens inputs correctly."""
+    nat_bot_chain = NatBotChain.from_llm(FakeLLM(), objective="testing")
+    url = "foo" * 10000
+    browser_content = "foo" * 10000
+    output = nat_bot_chain.execute(url, browser_content)
+    assert output == "bar"
+
+
+def test_variable_key_naming() -> None:
+    """Test that natbot handles variable key naming correctly."""
+    nat_bot_chain = NatBotChain.from_llm(
+        FakeLLM(),
+        objective="testing",
+        input_url_key="u",
+        input_browser_content_key="b",
+        output_key="c",
+    )
+    output = nat_bot_chain.execute("foo", "foo")
+    assert output == "bar"