fix: optional request bodies across generated clients

[adamcavendish]

Summary

Fixes optional request body handling across generated clients, with the Rust generator now preserving requestBody.required through API planning and backend emission.

What changed

• Rust API signatures now emit Option<&T> for optional request bodies and keep &T for required bodies.
• Rust reqwest and aioduct skip body construction entirely when an optional body is None.
• Rust ureq sends an empty body for absent optional body-capable requests, matching ureq/OkHttp-style constraints around body-capable POST flows.
• Optional unsupported multipart bodies now error only when a body is actually provided.
• Added focused optional non-multipart body goldens across Rust, Go, Java, Kotlin, Python, and TypeScript.
• Regenerated Rust multipart edge-case goldens for the optional multipart behavior.

Validation

• cargo test --test golden_tests_rust_reqwest test_optional_request_bodies_golden -- --nocapture
• cargo test --test golden_tests_rust_ureq test_optional_request_bodies_golden -- --nocapture
• cargo test --test golden_tests_rust_aioduct test_optional_request_bodies_golden -- --nocapture
• cargo test --test golden_tests_go_http test_optional_request_bodies_golden -- --nocapture
• cargo test --test golden_tests_java_okhttp test_optional_request_bodies_golden -- --nocapture
• cargo test --test golden_tests_kotlin_okhttp test_optional_request_bodies_golden -- --nocapture
• cargo test --test golden_tests_python_requests test_optional_request_bodies_golden -- --nocapture
• cargo test --test golden_tests_python_httpx test_optional_request_bodies_golden -- --nocapture
• cargo test --test golden_tests_typescript_fetch test_optional_request_bodies_golden -- --nocapture
• cargo test --test multipart_runtime_smoke -- --nocapture
• cargo fmt --check
• git diff --check
• Focused compile check for generated Rust optional-request-bodies packages for reqwest, ureq, and aioduct.