partners: Add Perplexity Chat Integration (#30618)

Perplexity's importance in the space has been growing, so we think it's
time to add an official integration!

Note: following the release of `langchain-perplexity` to `pypi`, we
should be able to add `perplexity` as an extra in
`libs/langchain/pyproject.toml`, but we're blocked by a circular import
for now.

---------

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>
Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/partners/perplexity/tests/integration_tests/test_chat_models_standard.py

@@ -0,0 +1,27 @@
+"""Standard LangChain interface tests."""
+
+from typing import Type
+
+import pytest
+from langchain_core.language_models import BaseChatModel
+from langchain_tests.integration_tests import ChatModelIntegrationTests
+
+from langchain_perplexity import ChatPerplexity
+
+
+class TestPerplexityStandard(ChatModelIntegrationTests):
+    @property
+    def chat_model_class(self) -> Type[BaseChatModel]:
+        return ChatPerplexity
+
+    @property
+    def chat_model_params(self) -> dict:
+        return {"model": "sonar"}
+
+    @pytest.mark.xfail(reason="TODO: handle in integration.")
+    def test_double_messages_conversation(self, model: BaseChatModel) -> None:
+        super().test_double_messages_conversation(model)
+
+    @pytest.mark.xfail(reason="Raises 400: Custom stop words not supported.")
+    def test_stop_sequence(self, model: BaseChatModel) -> None:
+        super().test_stop_sequence(model)

libs/partners/perplexity/tests/integration_tests/test_compile.py

@@ -0,0 +1,7 @@
+import pytest  # type: ignore[import-not-found]
+
+
+@pytest.mark.compile
+def test_placeholder() -> None:
+    """Used for compiling integration tests without running any real tests."""
+    pass