Lock OAS 3.1 type deserializer behavior and drop a dead null check

[jaipilot]

Companion improvement for the JAIPilot Cloud mirror of swagger-api#5266 (exact head 3ee7d00).

What the original PR does: adds Schema31Mixin.TypeDeserializer, the inverse of the existing TypeSerializer, so an OAS 3.1 'type' property serialized as a scalar string or a string array can be read back into the model's Set types field; wires it via @JsonDeserialize on Schema31Mixin.getTypes() and on GenericOpenApiContext.SortedSchemaMixin31.getTypes() (which reuses the same TypeDeserializer class rather than duplicating it); and adds JsonSchemaTypeRoundTripTest and SortedSchemaMixin31TypeRoundTripTest to prove the round trip.

What this companion adds on top, without changing any observable behavior:

1. Behavior lock: added one characterization test, typeArrayWithLiteralJsonNullElementSkipsIt, deserializing {"type":["string",null]} to explicitly cover the array branch's handling of a literal JSON null element (distinct from the string type name "null"). Ran it against the untouched PR-head implementation first (passed), matching this workflow's behavior_baseline/behavior_candidate requirement.
2. Cleanup: removed the unreachable 'n != null' reference-null guard inside TypeDeserializer's array-iteration lambda. JsonNode array iteration never yields an actual Java null reference (a JSON null literal becomes a NullNode instance), which was confirmed with a small standalone Jackson program, so the check was dead defensive clutter; the meaningful n.isNull() check is preserved unchanged.
3. Performance and modernization: evaluated and found not applicable. The deserializer is a small, non-hot-path operation with no build/runtime version surface in the changed files.

Verification: focused JsonSchemaTypeRoundTripTest and SortedSchemaMixin31TypeRoundTripTest pass identically before and after the edit; the full swagger-core (771 tests) and swagger-integration (3 tests) suites pass clean on the final candidate.

Limitations: performance was not benchmarked because the change is not on a measurable hot path (deterministic, non-looping allocation); no build/dependency file was touched so modernization was out of scope for this PR's changed paths.

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Generated by JAIPilot Cloud for #1 from Anthropic session sesn_01HyJAA8Nve7VizsrfD6wxAR.