Fix voluptuous-openapi LazySchema unhashable type bug

[allenporter]

Rationale

This PR addresses a TypeError: cannot use 'voluptuous_openapi.LazySchema' as a dict key bug that occurs when converting voluptuous schemas containing references back to OpenAPI. It also introduces proper reference denormalization and cycle-breaking mechanisms during serialization.

Proposed Changes

1. LazySchema Hashability: Implemented a __hash__ method in LazySchema returning hash(self.ref) to satisfy Python's hash contract while keeping the mutable root_schema dictionary out of the hash calculation.
2. Robust Callable Inspection: Updated the type hint extraction in convert() to check explicitly whether the callable is a class/routine or a custom callable instance (such as LazySchema), avoiding TypeError exceptions during type hint inspection.
3. Reference Denormalization:
   • When converting LazySchema objects back to OpenAPI, we now resolve and fully denormalize the target schema, rather than returning an incomplete/unresolvable {"$ref": ...} block without definitions.
   • Added a private _seen_refs: set[str] parameter to convert() to track active recursion paths.
   • Circular references/cycles are broken at the recursive boundary by returning {} (representing Any, which defaults to a string type via ensure_default), preventing infinite recursion.
4. Testing:
   • Added test_convert_schema_with_reference to verify denormalization and validation of simple references.
   • Added test_convert_schema_with_nested_reference to verify denormalization two levels deep.
   • Added test_convert_recursive_schema to verify cycle-breaking and recursion safety.
   • Added test_convert_custom_callable_class_instance to verify that custom callable class instances are converted robustly.