Add ability for markdown file browser to display json files

[brimock]

Why are these changes needed?

Enable the MarkdownFileBrowser to display '.json' files, such as a package.json file of a react package.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation 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.

[brimock]

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[afourney]

This will work as-is, and I'll approve it.

Note however that the next version of MarkItDown will support json files, and so we may revisit this.

[codecov]

Codecov Report

Attention: Patch coverage is 18.51852% with 22 lines in your changes missing coverage. Please review.

Project coverage is 75.49%. Comparing base (92d857f) to head (26c22e1).

Files with missing lines	Patch %	Lines
...n_ext/agents/file_surfer/_markdown_file_browser.py	18.51%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5463      +/-   ##
==========================================
- Coverage   75.58%   75.49%   -0.09%     
==========================================
  Files         189      189              
  Lines       12669    12688      +19     
==========================================
+ Hits         9576     9579       +3     
- Misses       3093     3109      +16     

Flag	Coverage Δ
unittests	75.49% <18.51%> (-0.09%)	⬇️

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[jackgerrits]

@brimock are you interested in continuing this? There are some CI failures that need to be addressed

[brimock]

@brimock are you interested in continuing this? There are some CI failures that need to be addressed

yes I am still interested, I can look into fixing it this weekend