fix: keep polymorphic subtype composition when re-resolved standalone (#5028)

[seonwooj0810]

Fixes #5028

Root cause

Polymorphic subtypes lose their allOf reference to the parent (regression in 2.2.41): a subtype that resolved to

"DateAttributeTypeImpl": { "type": "object", "allOf": [ { "$ref": "#/components/schemas/AttributeType" } ] }

degrades to { "type": "object", "properties": {} }.

The parent's resolveSubtypes builds each subtype's composed schema (allOf → parent) and registers it via defineModel. However, when the same subtype is resolved standalone afterwards — here while resolving the parent's @DiscriminatorMapping, whose resolveDiscriminator calls context.resolve(new AnnotatedType().type(mapping.schema())) without the subtype(true) flag — the resolution-cache lookup misses (AnnotatedType equality was tightened in #4975 / #5005), so the subtype is re-resolved as a plain object and modelByName is overwritten, dropping the composition.

Fix

Guard model registration in ModelConverterContextImpl so a schema without composition keywords (allOf/anyOf/oneOf) never overwrites an already-registered composed schema of the same name. The forward direction (plain → composed) is unaffected, so the composed subtype definition survives the later plain re-resolution. Applied at both storage sites (defineModel and the resolve() name mapping).

Tests

Added Ticket5028Test reproducing the reporter's hierarchy (interface parent with subTypes + discriminatorMapping, subtype via @Schema(implementation=...)). Both assertions fail on master and pass with the fix, for OpenAPI 3.0 and 3.1.

Verification done

• (1/2) No in-flight PR: searched open/all PRs for the symbol and my own open PRs — none cover this root cause; only cross-ref is a Copilot WIP bot issue.
• (4) Confirmed the overwrite path on master (resolveDiscriminator L2746 → ModelConverterContextImpl.resolve/defineName).
• (3) Regression source [Bug]: Regression: AnnotatedType equality change drops polymorphic subtype allOf entries #5003 is closed; this is the residual case.
• Full swagger-core module suite (722 tests) + focused composition/subtype/discriminator suite (38 tests) pass with the change. git commit -s (DCO).