Support non-JSON response content types (text/plain, application/octet-stream, etc.)

[halotukozak]

Description

Currently, response parsing only looks for application/json content type (SpecParser.kt:213). Responses with other content types like text/plain, application/octet-stream, application/xml, etc. are silently ignored — the response schema is set to null even if it's defined in the spec.

Acceptance criteria

• SpecParser recognizes at least text/plain and application/octet-stream response content types
• Generated client returns String for text responses and ByteArray for binary responses
• Response model carries content type information so the generator can emit correct deserialization
• JSON responses continue to work as before
• Tests cover parsing and generation for non-JSON response types