fixes the issues where exceptions from MCP server tools aren't serial… (#6482)

…ized properly

## Why are these changes needed?
The exceptions thrown by MCP server tools weren't being serialized
properly - the user would see `[{}, {}, ... {}]` instead of an actual
error/exception message.

## Related issue number

Fixes #6481 

## 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.
- [x] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [x] I've made sure all auto checks have passed.

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Signed-off-by: Peter Jausovec <peter.jausovec@solo.io>
Co-authored-by: Victor Dibia <victordibia@microsoft.com>
Co-authored-by: Victor Dibia <victor.dibia@gmail.com>