community[minor]: add ChatSnowflakeCortex chat model (#21490)

**Description:** This PR adds a chat model integration for [Snowflake
Cortex](https://docs.snowflake.com/en/user-guide/snowflake-cortex/llm-functions),
which gives an instant access to industry-leading large language models
(LLMs) trained by researchers at companies like Mistral, Reka, Meta, and
Google, including [Snowflake
Arctic](https://www.snowflake.com/en/data-cloud/arctic/), an open
enterprise-grade model developed by Snowflake.

**Dependencies:** Snowflake's
[snowpark](https://pypi.org/project/snowflake-snowpark-python/) library
is required for using this integration.

**Twitter handle:** [@gethouseware](https://twitter.com/gethouseware)

- [x] **Add tests and docs**:
1. integration tests:
`libs/community/tests/integration_tests/chat_models/test_snowflake.py`
2. unit tests:
`libs/community/tests/unit_tests/chat_models/test_snowflake.py`
  3. example notebook: `docs/docs/integrations/chat/snowflake.ipynb`


- [x] **Lint and test**: Run `make format`, `make lint` and `make test`
from the root of the package(s) you've modified. See contribution
guidelines for more: https://python.langchain.com/docs/contributing/

libs/community/tests/unit_tests/chat_models/test_imports.py

@@ -51,6 +51,7 @@ EXPECTED_ALL = [
     "QianfanChatEndpoint",
     "VolcEngineMaasChat",
     "ChatOctoAI",
+    "ChatSnowflakeCortex",
 ]
 
 

libs/community/tests/unit_tests/chat_models/test_snowflake.py

@@ -0,0 +1,24 @@
+"""Test ChatSnowflakeCortex."""
+
+from langchain_core.messages import AIMessage, HumanMessage, SystemMessage
+
+from langchain_community.chat_models.snowflake import _convert_message_to_dict
+
+
+def test_messages_to_prompt_dict_with_valid_messages() -> None:
+    messages = [
+        SystemMessage(content="System Prompt"),
+        HumanMessage(content="User message #1"),
+        AIMessage(content="AI message #1"),
+        HumanMessage(content="User message #2"),
+        AIMessage(content="AI message #2"),
+    ]
+    result = [_convert_message_to_dict(m) for m in messages]
+    expected = [
+        {"role": "system", "content": "System Prompt"},
+        {"role": "user", "content": "User message #1"},
+        {"role": "assistant", "content": "AI message #1"},
+        {"role": "user", "content": "User message #2"},
+        {"role": "assistant", "content": "AI message #2"},
+    ]
+    assert result == expected