fix(wit-bindgen-rust): derive async-func names from item_name

[rvolosatovs]

wit-parser prefixes async functions in func.name: [async]name, [async method]…, [async static]…. Four generated names must agree for the stubs to compile: the client free-fn, the Handler trait method, the serve-dispatch Handler::… call, and the wire/rpc name.

For freestanding async funcs two of those sites derived the Rust identifier from the prefixed func.name via to_rust_ident(&func.name), which falls through to heck::to_snake_case and turns [async]one-argument into async_one_argument:

• the client free-fn name (the use_item_name == false branch of print_docs_and_params), and
• the serve-dispatch Handler::… call (the freestanding arm reused the prefixed local name).

The Handler trait method and the wire name (rpc_func_name) already used the prefix-stripped item name, so for an async func the names drifted and the bindings failed to compile (E0432 on the client free-fn import, E0782 on the serve glue's Handler::async_… call). The async func modifier is meant to be a no-op in wRPC, where every func is already async over the transport.

Switch both sites to func.item_name() (identical to func.name for non-async functions), so all four names derive from the same prefix-stripped item name. Resource async methods/statics already used item_name() everywhere and were not affected; the internal serve_interface local bindings (f_…/m_…/s_…) keep deriving from func.name so same-named methods on different resources stay distinct.

The Go generator is unaffected: identifiers go through the centralized go_func_name and the wire name through rpc_func_name, both of which strip all prefixes.

Add regression coverage exercising the async func modifier:

• tests/runtime/rust/async-modifier: freestanding async funcs exercised client↔server over the transport (fails to compile before this change).
• tests/codegen/async-resource.wit: compile-only coverage of async func freestanding plus [async method]/[async static]/constructor on a resource.

Assisted-by: claude:claude-opus-4-8