Parse backtick-enclosed json (#6607)

## Why are these changes needed?

Some models enclose json in markdown code blocks

## Related issue number

resolves https://github.com/microsoft/autogen/issues/6599. , #6547 

## Checks

- [ ] I've included any doc changes needed for
<https://microsoft.github.io/autogen/>. See
<https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md> to
build and test documentation locally.
- [ ] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [ ] I've made sure all auto checks have passed.

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Co-authored-by: Victor Dibia <victordibia@microsoft.com>