fix: adds missing import for gemini

[abossenbroek]

Why are these changes needed?

When using Gemini as LLM client, an error was reported caused by a missing package installation. Due to this error it was not possible to use Gemini as a LLM client. The bug is resolved by adding jsonschema as dependency.

Related issue number

No separate ticket was opened for this issue.

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation to build and test documentation locally. [Couldn't find applicable documentation]
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

Local test

On a fresh venv, before the patch the behavior when installing with [gemini] was,

after the fix, the behavior is,

[marklysze]

@abossenbroek, thanks so much... I've tested the pip install gemini and it works without exception, thanks!

Once tests go through, we can merge!