Support forwarding parameters in the Rust bindings

[ksss]

Problem

f9f223c ("Support forwarding parameters in method types") added rbs_types_function_forwarding_param_t and rbs_types_function_t.forwarding to the C AST, but the Rust crates were not updated. From a clean build, ruby-rbs fails:

• E0425: rbs_types_function_forwarding_param_t not found (missing from the bindgen allowlist)
• E0004: Node::FunctionForwardingParam is not handled in convert.rs

This stayed hidden because ruby-rbs-sys was not rebuilt after C header changes (see #3198).

Changes

• ruby-rbs-sys: allowlist rbs_types_function_forwarding_param_t, rbs_parser_new_with_options and rbs_parser_options_t.
• ruby-rbs: add ast::ForwardingParam (location only, like RBS::Types::Function::ForwardingParam) and FunctionType::forwarding, converted from the new node.
• Add a crate-private ParseOptions / parse_with_options. The parser rejects (...) unless enable_forwarding_params is set, so the conversion could not be tested through parse. It is pub(crate) on purpose; whether to expose it publicly is left for a separate decision.

Verification

LC_ALL=en_US.UTF-8, in rust/:

• cargo build, cargo test, cargo clippy --all-targets, cargo fmt --check all pass.
• New test converts_forwarding_param checks def bar: (Integer, ...) -> void yields forwarding with a location spanning ..., and that (String) -> void yields None. Before the change, the same source fails with "forwarding parameter syntax is not enabled".

🤖 Generated with Claude Code

[ksss]

Closing: this was based on a wrong premise. CI vendors the C sources from the pinned tag in rust/rbs_version (v4.1.2) via rake rust:rbs:sync, and forwarding params are not in v4.1.2, so the Rust crates are not actually out of sync. They will be updated together with the next pin bump.