fix regex pattern of structured output parser (#12929)
- **Description:** fix the regex pattern of
[StructuredChatOutputParser](https://github.com/langchain-ai/langchain/blob/master/libs/langchain/langchain/agents/structured_chat/output_parser.py#L18)
and add unit tests for the code change.
- **Issue:** #12158 #12922
- **Dependencies:** None
- **Tag maintainer:**
- **Twitter handle:** @hmdev3
- **NOTE:** This PR conflicts #7495 . After #7495 is merged, I am going
to update PR.
