[BUG] Swagger 2.0 unresolved schema references are not detected during client generation

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When generating a client from a Swagger 2.0 specification containing a $ref to a schema that is not defined, OpenAPI Generator completes successfully instead of reporting an unresolved reference.

The generated library contains references to the missing model, but no corresponding model file is generated.

openapi-generator version

v7.25.0

OpenAPI declaration file content or url

swagger: '2.0'
info:
  title: Undefined Schema Reproduction
  version: '1.0.0'

paths:
  /test:
    post:
      parameters:
        - name: body
          in: body
          required: true
          schema:
            $ref: '#/definitions/DoesNotExist'
      responses:
        '200':
          description: OK

Generation Details

The issue appears to be language-independent

Steps to reproduce

Generate a client library using the above schema and inspect the results.

Related issues/PRs

Suggest a fix

When a schema definition is missing an error should be raised as is done for 3.0.x declarations

Reproduce client generation with the provided Swagger 2.0 YAML and inspect how unresolved schema references are handled for Swagger 2.0 versus 3.0 declarations. Trace the generator's reference-resolution and validation entry points, then add coverage showing that a missing definition raises an error instead of producing a client with a missing model.