reopened from #4125

This PR improves the JSON parsing logic in StructuredOutputParser.ts to handle malformed JSON responses from LLMs more effectively.

Changes:
• Fixed JSON extraction logic:
• Previously, JSON extraction relied on splitting text using code block markers (`json).
• Now, it uses a regex (/(?:^[^{[])|(?:[^}\]]$)/g) to remove any leading or trailing non-JSON content, ensuring a more robust parsing approach.

Why this change?
• The previous approach failed in some edge cases where JSON responses did not follow the expected format.
• The new regex-based approach ensures that only valid JSON content is passed to jsonrepair, reducing the likelihood of parsing errors.

Testing:
• Manually tested with different malformed JSON outputs to verify correct parsing.
• Ensured existing functionality remains intact.

Do you have some examples where the old approach is not able to parse, but solvable with the newer approach?

yes, basically it breaks if llm outputs something like this:

sure, here's you json output:

{
  "result": "string"

in our case it was even worse - we provided few shot examples and it repeated part of the prompt and only then provided the json response