community: fix AttributeError: 'YandexGPT' object has no attribute '_grpc_metadata' (#24432)

Fixes #24049

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

libs/community/tests/unit_tests/llms/test_yandex.py

@@ -0,0 +1,38 @@
+import pytest
+
+from langchain_community.llms.yandex import YandexGPT
+
+
+def test_yandexgpt_initialization() -> None:
+    llm = YandexGPT(
+        iam_token="your_iam_token",  # type: ignore[arg-type]
+        api_key="your_api_key",  # type: ignore[arg-type]
+        folder_id="your_folder_id",
+    )
+    assert llm.model_name == "yandexgpt-lite"
+    assert llm.model_uri.startswith("gpt://your_folder_id/yandexgpt-lite/")
+
+
+def test_yandexgpt_model_params() -> None:
+    llm = YandexGPT(
+        model_name="custom-model",
+        model_version="v1",
+        iam_token="your_iam_token",  # type: ignore[arg-type]
+        api_key="your_api_key",  # type: ignore[arg-type]
+        folder_id="your_folder_id",
+    )
+    assert llm.model_name == "custom-model"
+    assert llm.model_version == "v1"
+    assert llm.iam_token.get_secret_value() == "your_iam_token"
+    assert llm.model_uri == "gpt://your_folder_id/custom-model/v1"
+
+
+def test_yandexgpt_invalid_model_params() -> None:
+    with pytest.raises(ValueError):
+        YandexGPT(model_uri="", iam_token="your_iam_token")  # type: ignore[arg-type]
+    with pytest.raises(ValueError):
+        YandexGPT(
+            iam_token="",  # type: ignore[arg-type]
+            api_key="your_api_key",  # type: ignore[arg-type]
+            model_uri="",
+        )