fix(jsonschema): embed relations of non-resource objects in output schema

[alexisLefebvre]

Q	A
Branch?	4.3
Bug fix?	yes
New feature?	no
Deprecations?	no
Issues	-
License	MIT
Doc PR	-

What

On output, a relation declared on a non-resource object (e.g. a Doctrine entity embedded as a readable link, but not itself an #[ApiResource]) is now embedded as a $ref in the generated JSON Schema, instead of being typed as an iri-reference string.

Why

A non-resource object is serialized by Symfony's standard object normalizer — AbstractItemNormalizer::supportsNormalization() only handles resource classes — which embeds its related resources as full objects regardless of their readableLink or gen_id. SchemaPropertyMetadataFactory decided embed-vs-IRI per property and, for such a relation (default readableLink: false, and a real IRI so gen_id is not false), emitted an iri-reference string. The advertised schema therefore diverged from the actual payload, and a strict JSON Schema client rejected it (for instance an MCP client validating a tool's structuredContent against its advertised outputSchema, where an embedded OrderItem.supplier object failed {type: string, null}).

The fix mirrors the serializer at the single factory that owns the decision: on output, when the parent class is not a resource, treat its resource relations as embedded. Relations on resource DTOs keep following their own readableLink and are unaffected — covered by an added test that pins a resource parent to iri-reference.