Client SDK generation fails when referenced domain contains schema with pattern

Description

Client SDK generation fails when referenced domain contains schema with pattern

If a domain contains an schema like this:

components:
  schemas:
    countryCode:
      description: "An ISO 3166-1 Alpha-2 country code."
      type: 'string'
      pattern: '([A-Z]){2}' # inclusion of this line causes Client SDK generation to fail.
      example: 'US'

and the domain schema is referenced in a API spec like this:

paths:
  /example/path:
    get:
      responses:
        '200':
          content:
            application/json:
              schema:
                $ref: 'https://example.com/path/to/domain#/components/schemas/countryCode'

Client SDK generation will fail with the following error:

Error.There was an error generating the clients csharp code. Please ensure that the definition and all external refs (inc. domains) are valid.

Commenting out the pattern will allow the SDK to generate as expected.

Swagger-codegen version

Whichever version is in current use by SwaggerHub. I assume it is current.

Swagger declaration file content or url

See description.

Command line used for generation

Whatever SwaggerHub uses when selecting the option to generate Client SDKs for an API.

Steps to reproduce

See description.

Related issues/PRs

Not to my knowledge.

Suggest a fix/enhancement

Unknown.

The report names no repository files, tests, or entry points. Start with the minimal OpenAPI YAML containing the externally referenced schema and its pattern, then reproduce C# client generation through the applicable command or entry point. Done means generation succeeds and the pattern constraint is handled correctly.