[BUG][Spring] Named schemas referenced only inside allOf compositions are not generated as standalone model classes

Description:

When using the spring generator with OAS 3.1.0, named schemas that are only referenced from within an allOf composition (not directly from an operation response or another top-level schema property) are not generated as standalone Java classes. The generator creates inline classes that attempt to reference these missing classes, causing compilation failures.

openapi-generator version: 7.21.0

Generation target: spring (spring-boot library)

Minimal reproducible spec:

openapi: "3.1.0"
info:
  title: Test API
  version: 1.0.0
servers:
  - url: /api
paths:
  /items:
    get:
      operationId: getItems
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ItemListResponse'
components:
  schemas:
    BaseResponse:
      type: object
      required:
        - txnId
        - timestamp
      properties:
        txnId:
          type: string
        timestamp:
          type: string
          format: date-time
      discriminator:
        propertyName: txnId

    ItemListResponse:
      type: object
      allOf:
        - $ref: '#/components/schemas/BaseResponse'
        - type: object
          properties:
            items:
              type: array
              items:
                $ref: '#/components/schemas/Item'

    Item:
      type: object
      required:
        - details
        - subItems
      properties:
        id:
          type: string
        details:
          type: array
          items:
            $ref: '#/components/schemas/ItemDetail'
        subItems:
          type: array
          items:
            $ref: '#/components/schemas/Item'

    ItemDetail:
      type: object
      properties:
        name:
          type: string
        value:
          type: string

Command:

java -jar openapi-generator-cli-7.21.0.jar generate \
  -i spec.yaml \
  -g spring \
  --library spring-boot \
  --additional-properties=useJakartaEe=true,useSpringBoot3=true,interfaceOnly=true,delegatePattern=true

Expected behavior:

The generator should produce standalone Java classes for all named schemas: BaseResponse, ItemListResponse, Item, and ItemDetail.

Actual behavior:

Item.java is NOT generated
ItemDetail.java is NOT generated
The generator creates an inline class GetItems200ResponseAllOfItemsInner.java that imports and extends Item, but Item.java doesn't exist
Similarly, GetItems200ResponseAllOfItemsInnerDetailsInner.java references ItemDetail which doesn't exist
Compilation fails with cannot find symbol errors
Key observation:

Item and ItemDetail are only referenced from within the allOf inline composition in ItemListResponse. They are not directly referenced by any operation response or top-level property. If Item is also referenced directly (e.g., as a standalone response schema on another endpoint), the generator produces it correctly.

Workarounds attempted (none resolved):

--inline-schema-name-mappings — mappings not applied
schemaMappings — no effect
openapiNormalizer REF_AS_PARENT_IN_ALLOF=true — no effect
true config option — no effect

Related issues: #1123, #21006, #11323

Start by running the minimal OAS 3.1 spec with the documented Spring generator command, then inspect schema/model discovery and allOf handling in the Spring generator. Add a regression case covering BaseResponse, ItemListResponse, Item, and ItemDetail, and verify that all four standalone classes are generated and compile without missing-symbol errors.