[BUG] sanitize_for_json crashes on circular references (RecursionError) #189

Description

sanitize_for_json in server.py:74-83 recursively walks objects but has no cycle detection. If an sktime estimator contains circular references in its attributes (some do via parent/child pointers), this causes RecursionError and crashes the MCP server.

def sanitize_for_json(obj):
    if isinstance(obj, dict):
        return {str(k): sanitize_for_json(v) for k, v in obj.items()}  # no cycle check
    elif isinstance(obj, (list, tuple)):
        return [sanitize_for_json(item) for item in obj]  # no cycle check

The hasattr(obj, "__dict__") fallback on line 81 catches most non-dict/list objects by converting to str(), but dict/list cycles are uncaught.

How to reproduce

Any tool returning a result containing a circular reference will trigger this. Likely scenario: an estimator with a parent attribute pointing back to a container.

Suggested fix

Track visited objects by id():

def sanitize_for_json(obj, _seen=None):
    if _seen is None:
        _seen = set()
    obj_id = id(obj)
    if obj_id in _seen:
        return str(obj)
    _seen.add(obj_id)
    if isinstance(obj, dict):
        return {str(k): sanitize_for_json(v, _seen) for k, v in obj.items()}
    ...

Alternatively, wrap the top-level call in try/except RecursionError.

Found while reading the codebase for PR #114 work.