fix: sanitize non-serializable exceptions in Pydantic ctx (#86)

[littlehongman]

This PR addresses issue #86.

Pydantic error objects may include a raw Python exception inside ctx["error"], which breaks JSON serialization. This update replaces the exception instance with a JSON-safe structure:

{"type": type(exc).__name__, "message": str(exc)}

All other ctx fields remain unchanged. No functional behavior is altered.

[stv8]

hey! is there any way you can add a small test for this?

[littlehongman]

hey! is there any way you can add a small test for this?

Just added. Thanks