comminity[patch]: fix #25575 YandexGPTs for _grpc_metadata (#25617)

it fixes two issues:

### YGPTs are broken #25575

```
File ....conda/lib/python3.11/site-packages/langchain_community/embeddings/yandex.py:211, in _make_request(self, texts, **kwargs)
..
--> 211 res = stub.TextEmbedding(request, metadata=self._grpc_metadata)  # type: ignore[attr-defined]

AttributeError: 'YandexGPTEmbeddings' object has no attribute '_grpc_metadata'
```
My gut feeling that #23841 is the cause.

I have to drop leading underscore from `_grpc_metadata` for quickfix,
but I just don't know how to do it _pydantic_ enough.

### minor issue:

if we use `api_key`, which is not the best practice the code fails with 

```
File ~/git/...../python3.11/site-packages/langchain_community/embeddings/yandex.py:119, in YandexGPTEmbeddings.validate_environment(cls, values)
...

AttributeError: 'tuple' object has no attribute 'append'
```

- Added new integration test. But it requires YGPT env available and
active account. I don't know how int tests dis\enabled in CI.
 - added small unit tests with mocks. Should be fine.

---------

Co-authored-by: mikhail-khludnev <mikhail_khludnev@rntgroup.com>

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

@@ -1,3 +1,7 @@
+import os
+from unittest import mock
+from unittest.mock import MagicMock
+
 import pytest
 
 from langchain_community.llms.yandex import YandexGPT
@@ -36,3 +40,53 @@ def test_yandexgpt_invalid_model_params() -> None:
             api_key="your_api_key",  # type: ignore[arg-type]
             model_uri="",
         )
+
+
+@pytest.mark.parametrize(
+    "api_key_or_token", [dict(api_key="bogus"), dict(iam_token="bogus")]
+)
+@pytest.mark.parametrize(
+    "disable_logging",
+    [dict(), dict(disable_request_logging=True), dict(disable_request_logging=False)],
+)
+@mock.patch.dict(os.environ, {}, clear=True)
+def test_completion_call(api_key_or_token: dict, disable_logging: dict) -> None:
+    absent_yandex_module_stub = MagicMock()
+    grpc_mock = MagicMock()
+    with mock.patch.dict(
+        "sys.modules",
+        {
+            "yandex.cloud.ai.foundation_models.v1."
+            "text_common_pb2": absent_yandex_module_stub,
+            "yandex.cloud.ai.foundation_models.v1.text_generation."
+            "text_generation_service_pb2": absent_yandex_module_stub,
+            "yandex.cloud.ai.foundation_models.v1.text_generation."
+            "text_generation_service_pb2_grpc": absent_yandex_module_stub,
+            "grpc": grpc_mock,
+        },
+    ):
+        grpc_mock.RpcError = Exception
+        stub = absent_yandex_module_stub.TextGenerationServiceStub
+        request_stub = absent_yandex_module_stub.CompletionRequest
+        msg_constructor_stub = absent_yandex_module_stub.Message
+        args = {"folder_id": "fldr", **api_key_or_token, **disable_logging}
+        ygpt = YandexGPT(**args)
+        grpc_call_mock = stub.return_value.Completion
+        msg_mock = mock.Mock()
+        msg_mock.message.text = "cmpltn"
+        res_mock = mock.Mock()
+        res_mock.alternatives = [msg_mock]
+        grpc_call_mock.return_value = [res_mock]
+        act_emb = ygpt.invoke("nomatter")
+        assert act_emb == "cmpltn"
+        assert len(grpc_call_mock.call_args_list) == 1
+        once_called_args = grpc_call_mock.call_args_list[0]
+        act_model_uri = request_stub.call_args_list[0].kwargs["model_uri"]
+        act_text = msg_constructor_stub.call_args_list[0].kwargs["text"]
+        act_metadata = once_called_args.kwargs["metadata"]
+        assert "fldr" in act_model_uri
+        assert act_text == "nomatter"
+        assert act_metadata
+        assert len(act_metadata) > 0
+        if disable_logging.get("disable_request_logging"):
+            assert ("x-data-logging-enabled", "false") in act_metadata