fix(langchain): Fix structured parser with triple backticks, adds tests

[johnguirgis]

Fixes #6734

Corrects an issue where the output parser naïvely attempts to parse contents of triple backticks as JSON.

1. Output parser falls back to parse the entire string when extracting from within triple backticks fails
2. Adds tests to account for different backtick positions in JSONs

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
langchainjs-docs	✅ Ready (Inspect)	Visit Preview		Nov 15, 2024 3:07am

1 Skipped Deployment

Name	Status	Preview	Comments	Updated (UTC)
langchainjs-api-refs	⬜️ Ignored (Inspect)			Nov 15, 2024 3:07am

[harrisbchong]

To maintain backwards compatibility, we modified our solution attempt at #6734 to leave the original parsing and behaviour in the first try block. Instead, we move our new parsing code inside the catch block, ensuring it executes only when the original code fails. The logic within the new try block is as follows: we know parsing the contents within the brace has failed, meaning that the only hope is to parse the string as whole (while trimming the leading or trailing whitespace).

[jacoblee93]

Thanks, seems reasonable!