Add support for "format": "json" in JSON schemas

[Brian Fox (onematchfox)]

Why are these changes needed?

Supports deserialization of JSON schemas (e.g. in MCP tools) that make use of properties "type": "string", "format": "json".

Related issue number

Closes #6841

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.

[Brian Fox (onematchfox)]

@microsoft-github-policy-service agree company="Embark Studios AB"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.91%. Comparing base (3b139c6) to head (7c709d8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6846   +/-   ##
=======================================
  Coverage   79.91%   79.91%           
=======================================
  Files         235      235           
  Lines       18172    18172           
=======================================
  Hits        14523    14523           
  Misses       3649     3649           

Flag	Coverage Δ
unittests	79.91% <ø> (ø)

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