fix(transport): fix CI failures for unix socket transport

- Use std::io::Error::other() instead of Error::new(ErrorKind::Other)
  to satisfy clippy::io_other_error on newer nightly
- Use #[tokio::test(flavor = "current_thread")] for unix socket tests
  since axum's serve(UnixListener) requires spawn_local
- Gate validate_custom_header behind client-side-sse feature since it
  references http::HeaderName which isn't available with default features

Author: wpfleger96

crates/rmcp/src/transport/common/http_header.rs

@@ -17,6 +17,7 @@ pub(crate) const RESERVED_HEADERS: &[&str] = &[
 /// Checks whether a custom header name is allowed.
 /// Returns `Ok(())` if allowed, `Err(name)` if rejected as reserved.
 /// `MCP-Protocol-Version` is reserved but allowed through (the worker injects it post-init).
+#[cfg(feature = "client-side-sse")]
 pub(crate) fn validate_custom_header(name: &http::HeaderName) -> Result<(), String> {
     if RESERVED_HEADERS
         .iter()
@@ -93,24 +94,28 @@ mod tests {
         assert_eq!(extract_scope_from_header("Bearer"), None);
     }
 
+    #[cfg(feature = "client-side-sse")]
     #[test]
     fn validate_rejects_reserved_accept() {
         let name = http::HeaderName::from_static("accept");
         assert!(validate_custom_header(&name).is_err());
     }
 
+    #[cfg(feature = "client-side-sse")]
     #[test]
     fn validate_rejects_reserved_session_id() {
         let name = http::HeaderName::from_static("mcp-session-id");
         assert!(validate_custom_header(&name).is_err());
     }
 
+    #[cfg(feature = "client-side-sse")]
     #[test]
     fn validate_allows_mcp_protocol_version() {
         let name = http::HeaderName::from_static("mcp-protocol-version");
         assert!(validate_custom_header(&name).is_ok());
     }
 
+    #[cfg(feature = "client-side-sse")]
     #[test]
     fn validate_allows_custom_header() {
         let name = http::HeaderName::from_static("x-custom");

crates/rmcp/src/transport/common/unix_socket.rs

@@ -119,7 +119,7 @@ async fn connect_unix(socket_path: &str) -> Result<UnixStream, std::io::Error> {
             Ok::<_, std::io::Error>(stream)
         })
         .await
-        .map_err(|e| std::io::Error::new(std::io::ErrorKind::Other, e))??;
+        .map_err(std::io::Error::other)??;
         return UnixStream::from_std(std_stream);
     }
 

crates/rmcp/tests/test_unix_socket_transport.rs

@@ -107,7 +107,7 @@ async fn mcp_handler(
 }
 
 /// Integration test: MCP client connects and completes handshake over a Unix domain socket.
-#[tokio::test]
+#[tokio::test(flavor = "current_thread")]
 async fn test_unix_socket_mcp_handshake() -> anyhow::Result<()> {
     let dir = std::env::temp_dir().join(format!("rmcp-test-{}", std::process::id()));
     std::fs::create_dir_all(&dir)?;
@@ -162,7 +162,7 @@ async fn test_unix_socket_mcp_handshake() -> anyhow::Result<()> {
 }
 
 /// Integration test: Custom headers are sent through the Unix socket transport.
-#[tokio::test]
+#[tokio::test(flavor = "current_thread")]
 async fn test_unix_socket_custom_headers() -> anyhow::Result<()> {
     let dir = std::env::temp_dir().join(format!("rmcp-test-headers-{}", std::process::id()));
     std::fs::create_dir_all(&dir)?;
@@ -229,7 +229,7 @@ async fn test_unix_socket_custom_headers() -> anyhow::Result<()> {
 }
 
 /// Integration test: Convenience constructor `from_unix_socket` works end-to-end.
-#[tokio::test]
+#[tokio::test(flavor = "current_thread")]
 async fn test_unix_socket_convenience_constructor() -> anyhow::Result<()> {
     let dir = std::env::temp_dir().join(format!("rmcp-test-conv-{}", std::process::id()));
     std::fs::create_dir_all(&dir)?;