Return error for UDS connections on wasi

[Neol3108]

Motivation

Building a client for wasm32-wasip2 with the channel feature fails because Tonic imports tokio::net::UnixStream on every non-Windows target. Tokio only exposes this type on Unix targets. WASI is neither Windows nor Unix, so the import fails even when the client only uses TCP.

Solution

Use cfg(unix) for the Unix socket implementation and cfg(not(unix)) for the existing unsupported-platform fallback. This matches Tokio’s platform guards and keeps Unix socket requests returning an error on platforms that don’t support them.
Adds tests for Unix socket connections and the fallback, plus a WASI client compile check in CI. The 77 library tests, WASI compile check, Clippy, and formatting checks pass locally.

[linux-foundation-easycla]

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• ✅ login: Neol3108 / name: Noël Hagestein (8203a34)

[Neol3108]

Failing CI seems unrelated