fix: resolve RecursionError for self-referencing schemas

[mojixcoder]

Describe your changes

resolve_schema_references() would raise a RecursionError when the OpenAPI schema contained a self-referencing (recursive) Pydantic model — e.g. a CompoundFilter that contains a list of CompoundFilter | ComparisonFilter.

Root cause: The function unconditionally expanded every $ref it encountered, including back-references to a schema already being expanded, leading to infinite recursion.

Fix: Added a _visited parameter (a frozenset of $ref paths currently on the expansion stack). When a $ref is encountered that is already in _visited, the function returns the $ref in place rather than expanding it, breaking the cycle.

Test: Added test_self_referencing_schema_does_not_raise_recursion_error with a minimal recursive model (CompoundFilter → List[ComparisonFilter | CompoundFilter]) to guard against regressions.

Issue ticket number and link (if applicable)

Clsoes #287

Screenshots of the feature / bugfix

Checklist before requesting a review

• Added relevant tests
• Run ruff & mypy
• All tests pass

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