community[patch]: Update the default api_url and reqeust_body of sparkllm embedding (#22136)

- **Description:** When I was running the SparkLLMTextEmbeddings,
app_id, api_key and api_secret are all correct, but it cannot run
normally using the current URL.

    ```python
    # example
    from langchain_community.embeddings import SparkLLMTextEmbeddings

    embedding= SparkLLMTextEmbeddings(
        spark_app_id="my-app-id",
        spark_api_key="my-api-key",
        spark_api_secret="my-api-secret"
    )
    embedding= "hello"
    print(spark.embed_query(text1))
    ```

![sparkembedding](https://github.com/langchain-ai/langchain/assets/55082429/11daa853-4f67-45b2-aae2-c95caa14e38c)
   
So I updated the url and request body parameters according to
[Embedding_api](https://www.xfyun.cn/doc/spark/Embedding_api.html), now
it is runnable.

libs/community/tests/unit_tests/embeddings/test_sparkllm.py

@@ -0,0 +1,47 @@
+import os
+from typing import cast
+
+import pytest
+from langchain_core.pydantic_v1 import SecretStr, ValidationError
+
+from langchain_community.embeddings import SparkLLMTextEmbeddings
+
+
+def test_sparkllm_initialization_by_alias() -> None:
+    # Effective initialization
+    embeddings = SparkLLMTextEmbeddings(
+        app_id="your-app-id",  # type: ignore[arg-type]
+        api_key="your-api-key",  # type: ignore[arg-type]
+        api_secret="your-api-secret",  # type: ignore[arg-type]
+    )
+    assert cast(SecretStr, embeddings.spark_app_id).get_secret_value() == "your-app-id"
+    assert (
+        cast(SecretStr, embeddings.spark_api_key).get_secret_value() == "your-api-key"
+    )
+    assert (
+        cast(SecretStr, embeddings.spark_api_secret).get_secret_value()
+        == "your-api-secret"
+    )
+
+
+def test_initialization_parameters_from_env() -> None:
+    # Setting environment variable
+    os.environ["SPARK_APP_ID"] = "your-app-id"
+    os.environ["SPARK_API_KEY"] = "your-api-key"
+    os.environ["SPARK_API_SECRET"] = "your-api-secret"
+
+    # Effective initialization
+    embeddings = SparkLLMTextEmbeddings()
+    assert cast(SecretStr, embeddings.spark_app_id).get_secret_value() == "your-app-id"
+    assert (
+        cast(SecretStr, embeddings.spark_api_key).get_secret_value() == "your-api-key"
+    )
+    assert (
+        cast(SecretStr, embeddings.spark_api_secret).get_secret_value()
+        == "your-api-secret"
+    )
+
+    # Environment variable missing
+    del os.environ["SPARK_APP_ID"]
+    with pytest.raises(ValidationError):
+        SparkLLMTextEmbeddings()