fix: regenerate v3 client from updated schema

[QuentinN42]

Summary

• harden OpenAPI schema conversion so enum extraction handles mixed values like null and strings
• rewrite unsupported stability fields to x-stability before generation so the schema update script completes again
• regenerate the v3 client and add compatibility shims so the CLI still builds against renamed response models

Validation

• bash ./scripts/update-schema.sh
• go test ./...