fix(kosong): raise a clear error on circular $ref in deref_json_schema - #2506

Related Issue

None. This is a small self-contained bug fix, well under the 100-line guideline in CONTRIBUTING. Happy to open an issue first if you prefer that flow.

Description

kosong.utils.jsonschema.deref_json_schema inlines every local $ref and recursively traverses the target, but it has no cycle detection. A self-referential schema, which Pydantic emits for a recursive model, therefore recursed until a RecursionError, despite the docstring promising expansion "without infinite recursion". A tool whose parameter model is recursive crashes tool conversion with a stack overflow:

from pydantic import BaseModel
from kosong.utils.jsonschema import deref_json_schema

class Node(BaseModel):
    value: str
    children: list["Node"] = []

deref_json_schema(Node.model_json_schema())  # RecursionError

This change tracks the $ref targets currently being inlined along the active chain and raises a clear ValueError naming the circular reference when a $ref points back into that chain. Schemas that reference the same definition several times in different branches are unaffected, since the tracking is scoped to the active chain.

Adds a regression test (test_deref_circular_ref_raises); it fails with RecursionError on the current code. All 13 tests in test_json_schema_deref.py and the kimi tool-conversion tests pass, and ruff is clean.

Checklist

• I have read the CONTRIBUTING document.
• I have linked the related issue, if any. (No existing issue; explained above.)
• I have added tests that prove my fix is effective or that my feature works.
• I have run make gen-changelog to update the changelog. (Entry added to the Unreleased section by hand in the existing style; make gen-changelog drives Kimi CLI itself, which I could not run locally.)
• I have run make gen-docs to update the user documentation. (N/A: no user-facing docs change.)