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_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