docs: add Snowflake Cortex REST API integration (ChatOpenAI + ChatAnthropic)

[sfc-gh-nashukla (sfc-gh-nashukla)]

Title:

docs: add Snowflake Cortex REST API integration (ChatOpenAI + ChatAnthropic)

Description:

## Summary

- Add new `chat/cortex_rest_api.mdx` page documenting Snowflake Cortex REST API usage with LangChain via `ChatOpenAI` and `ChatAnthropic`
- Update `providers/snowflake.mdx` with full Snowflake + LangChain capabilities (9 integrations)
- Update `chat/index.mdx` to include Cortex REST API in the "Chat Completions API" section

## Details

Snowflake Cortex REST API supports two industry-standard API specifications:

| Compatibility | Endpoint | LangChain Class |
|---|---|---|
| OpenAI Chat Completions | `/api/v2/cortex/v1/chat/completions` | `ChatOpenAI` |
| Anthropic Messages | `/api/v2/cortex/v1/messages` | `ChatAnthropic` |

This is a zero-dependency approach — users only need `langchain-openai` or `langchain-anthropic` (which they likely already have) plus a Snowflake PAT.

The new chat integration page includes: setup, streaming, tool calling, structured output, and authentication options for both endpoints.

The updated provider page now highlights all current Snowflake + LangChain integrations including Cortex REST API, Cortex Search, Cortex Analyst, Cortex Agents, Cortex AI Tools, MCP Integration, Embeddings, and Document Loader.

## References

- [Cortex REST API documentation](https://docs.snowflake.com/en/user-guide/snowflake-cortex/cortex-rest-api)
- [langchain-snowflake package](https://pypi.org/project/langchain-snowflake/)

[github-actions]

Thanks for opening a docs PR, sfc-gh-nashukla (@sfc-gh-nashukla)! When it's ready for review, please add the relevant reviewers:

• @mdrxy (Python integrations)

[sfc-gh-nashukla (sfc-gh-nashukla)]

Ready for review. cc Mason Daugherty (@mdrxy) (Python integrations)