fix(sdk/rust): add Host header for Unix socket requests

The http-client-unix-domain-socket library requires a Host header for
HTTP/1.1 requests over Unix sockets. Without it, requests return 400
Bad Request.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: h4x3rotab

sdk/rust/src/dstack_client.rs

@@ -104,7 +104,7 @@ impl DstackClient {
                     .send_request_json::<_, _, Value>(
                         path,
                         Method::POST,
-                        &[("Content-Type", "application/json")],
+                        &[("Content-Type", "application/json"), ("Host", "dstack")],
                         Some(&payload),
                     )
                     .await?;