Expose hard-coded request fields through builder API and fix JSON null serialization

[luckenco]

Problem

Several request fields are hard-coded and not exposed through the builder, with some being serialized as JSON nulls.

Current Issues

• Hard-coded fields: instructions, user, max_tool_calls, store, truncation
• JSON null serialization: Fields not set are being serialized as null instead of omitted
• Missing builder API: No way to configure these important request parameters

Specific Locations

• src/responses/request.rs:18
• src/responses/request.rs:25
• src/responses/request.rs:48

Requirements

• Expose all hard-coded fields through the builder API
• Fix JSON serialization to omit unset fields (not send null)
• Add proper validation for new builder methods
• Maintain backward compatibility where possible
• Update documentation for new builder options

Impact

Users cannot configure important request parameters, and the API sends unnecessary null values in requests.

[luckenco]

disabled serialization for None fields in commit d4050361332.

[luckenco]

This issue has to be revisited, once completion API providers are implemented and the API for that is somewhat stable.