infer: emit $ref/$defs for recursive types instead of erroring

[rafaeljusto]

For and ForType previously returned "cycle detected for type ..." when a Go type referred to itself, directly or indirectly. This made it impossible to generate a schema for self-referential types such as jsonschema.Schema itself, which is a common need when accepting a JSON schema as input to an MCP tool.

Now, when a named type closes a cycle, its schema is emitted once under "$defs" and referenced with "$ref" wherever it recurs. The recursion bookkeeping (seen set, assigned def names, collected defs) is threaded through forType via a shared inferState. Output for non-recursive types is unchanged: no "$defs" is added and no "$ref" is emitted.

Verified end to end against the modelcontextprotocol/go-sdk, which depends on this library: ForType, Resolve{ValidateDefaults:true}, Validate, ApplyDefaults, the JSON wire round-trip, and AddTool all handle the generated recursive schemas, and the SDK's own test suite passes.

Resolves #46

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[rafaeljusto]

❌ a0abba9 Co-Author: <no****y​@anthropic.com>

Oh, looks like I cannot set the co-author thing due to the CLA rules (Claude Opus 4.8 (1M context) <noreply@anthropic.com>).

I will force-push again.

[karolpiotrowicz]

@wolo-lab — routing this to you for a direction call rather than a code review.

For and ForType currently return "cycle detected" for a self-referential Go type. This change emits the type once under $defs and references it with $ref wherever it recurs, so recursive types become expressible.

No exported signature changes, which is why it is easy to miss as a scope call. What changes is the shape of the schema these functions emit, and that is effectively an output contract: anything comparing, storing or serving generated schemas sees different JSON. The author states output for non-recursive types is unchanged and reports verifying end to end against modelcontextprotocol/go-sdk, which depends on this library — and adk-go depends on go-sdk, so the change reaches us transitively. That is the part worth a deliberate decision.

Open 72 days. Happy to run a full review once there is a direction.