#136 feat(python): configure model serialization

Add opt-in Python configuration schema entries for preserveUnknownFields and modelSerializationMethods. The first preserves undeclared JSON model fields; the second adds to_dict and to_json model helpers. Both default to disabled in the generator. Explicitly typed additional properties retain their validation.

Generator implementation: https://github.com/speakeasy-api/openapi-generation/pull/133

Validation: go test ./... passes.