Support self-referential structures

**Describe the bug**
When attempting to generate a jsonschema using a struct that references itself([jsonschema.Schema](https://github.com/google/jsonschema-go/blob/ad34a931e77508dc3d8c9a8abc92ec089001c7cf/jsonschema/schema.go#L42) for example), an error is returned: `cycle detected for type jsonschema.Schema`. This error is coming from [jsonschema/infer.go
](https://github.com/google/jsonschema-go/blob/ad34a931e77508dc3d8c9a8abc92ec089001c7cf/jsonschema/infer.go#L124)

**To Reproduce**
Steps to reproduce the behavior:
1. Generate a jsonschema using jsonschema.Schema or any other struct that references itself.

**Expected behavior**
Generate a ref instead of erroring

**Logs**
`cycle detected for type jsonschema.Schema`

**Additional context**
For reference, my use case is accepting a json schema as input to an MCP tool.