partners: Fixed the procedure of initializing pad_token_id (#29500)

- **Description:** Add to check pad_token_id and eos_token_id of model
config. It seems that this is the same bug as the HuggingFace TGI bug.
It's same bug as #29434
- **Issue:** #29431
- **Dependencies:** none
- **Twitter handle:** tell14

Example code is followings:
```python
from langchain_huggingface.llms import HuggingFacePipeline

hf = HuggingFacePipeline.from_model_id(
    model_id="meta-llama/Llama-3.2-3B-Instruct",
    task="text-generation",
    pipeline_kwargs={"max_new_tokens": 10},
)

from langchain_core.prompts import PromptTemplate

template = """Question: {question}

Answer: Let's think step by step."""
prompt = PromptTemplate.from_template(template)

chain = prompt | hf

question = "What is electroencephalography?"

print(chain.invoke({"question": question}))
```

libs/partners/huggingface/tests/integration_tests/test_standard.py

@@ -67,12 +67,14 @@ class TestHuggingFaceEndpoint(ChatModelIntegrationTests):
         super().test_bind_runnables_as_tools(model)
 
     @pytest.mark.xfail(reason=("Not implemented"))
-    def test_structured_output(self, model: BaseChatModel) -> None:
-        super().test_structured_output(model)
+    def test_structured_output(self, model: BaseChatModel, schema_type: str) -> None:
+        super().test_structured_output(model, schema_type)
 
     @pytest.mark.xfail(reason=("Not implemented"))
-    def test_structured_output_async(self, model: BaseChatModel) -> None:  # type: ignore[override]
-        super().test_structured_output(model)
+    async def test_structured_output_async(
+        self, model: BaseChatModel, schema_type: str
+    ) -> None:  # type: ignore[override]
+        super().test_structured_output(model, schema_type)
 
     @pytest.mark.xfail(reason=("Not implemented"))
     def test_structured_output_pydantic_2_v1(self, model: BaseChatModel) -> None: