Mask api key for Together LLM (#13981)
- **Description:** Add unit tests and mask api key for Together LLM
- **Issue:** the issue
https://github.com/langchain-ai/langchain/issues/12165 ,
- **Dependencies:** N/A
- **Tag maintainer:** ?,
- **Twitter handle:** N/A
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Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>
