Support `default` response in OpenAPI endpoint parsing

[halotukozak]

Description

ClientGenerator.kt:171 resolves the return type by filtering responses whose key starts with "2". The OpenAPI default response is ignored entirely. When an endpoint only defines a default response (without explicit 2xx), the generated function returns Unit even though a response schema exists.

Acceptance criteria

• ClientGenerator falls back to default response schema when no explicit 2xx response is defined
• When both 2xx and default are present, 2xx takes precedence
• Tests cover endpoints with only default response, and with both default + 2xx