[codex] Respect OpenAPI media types

[k-kinzal]

Summary

• Select request and response media types from the OpenAPI content map instead of always emitting JSON.
• Serialize generated payloads for JSON, text/*, and application/x-www-form-urlencoded media types.
• Add schema-level fake data generation for non-JSON payloads so media types unsupported by the operation-level faker still work.

Validation

• composer test
• composer lint