What does this PR do?

Fixes toJSONSchema() with target: "openapi-3.0" to use $defs and #/$defs/ refs instead of definitions and #/definitions/ for recursive schemas.

Root cause

The defsSegment logic in to-json-schema.ts and json-schema-processors.ts used a binary choice: $defs for draft-2020-12, definitions for everything else. This meant openapi-3.0 was getting definitions — a Swagger 2.0 / JSON Schema Draft 4 concept that is not a recognized keyword in OpenAPI 3.0 Schema Objects.

For a recursive schema like:

const Recursive = z.lazy(() => z.object({ child: Recursive.optional() }));
const Schema = z.object({ data: Recursive });
z.toJSONSchema(Schema, { target: "openapi-3.0" });

The output contained "$ref": "#/definitions/__schema0" and a "definitions" key, which broke tools like Redoc that strictly validate against the OAS 3.0 spec.

Fix

Inverted the condition: definitions is now only used for draft-04 and draft-07, while everything else (including openapi-3.0 and draft-2020-12) uses $defs. This is forward-compatible since OpenAPI 3.1 adopted JSON Schema 2020-12's $defs convention.

Changed in three places:

1. to-json-schema.ts line 250 — makeURI ref path segment
2. to-json-schema.ts lines 486-491 — finalize defs key placement
3. json-schema-processors.ts line 653 — registry toJSONSchema defs segment

Tests

Added a new test "recursive schema openapi-3.0 uses $defs" that verifies:

• The output does NOT have a definitions key
• The output DOES have a $defs key
• Refs use #/$defs/ prefix
• Full inline snapshot of expected output

All 3577 existing tests pass.

Related issue

Closes #5693