refactor: Extract shared MCP Stdio NDJSON parsing loop into mcp-stdio library

mcp-stdio/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "mcp-stdio"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+tokio = { version = "1.53.1", features = ["io-util"] }
+tracing = "0.1.44"

mcp-stdio/src/lib.rs

@@ -0,0 +1,40 @@
+use tokio::io::{AsyncBufReadExt, AsyncReadExt, BufReader};
+
+/// Reads an MCP (NDJSON or LSP Content-Length prefixed) message from a buffered async reader.
+/// Returns the raw JSON string payload if successful, or None on EOF or error.
+pub async fn read_mcp_message<R: tokio::io::AsyncRead + Unpin>(
+    stdin: &mut BufReader<R>,
+) -> Option<String> {
+    let mut length = 0;
+    loop {
+        let mut line = String::new();
+        if stdin.read_line(&mut line).await.unwrap_or(0) == 0 {
+            return None;
+        }
+
+        if line.starts_with('{') {
+            return Some(line.trim_end().to_string());
+        }
+
+        let line = line.trim_end();
+        if line.is_empty() {
+            break;
+        }
+        
+        let lower_line = line.to_lowercase();
+        if let Some(len_str) = lower_line.strip_prefix("content-length:") {
+            length = len_str.trim().parse().unwrap_or(0);
+        }
+    }
+    
+    if length == 0 {
+        return None;
+    }
+    
+    let mut buffer = vec![0; length];
+    if stdin.read_exact(&mut buffer).await.is_err() {
+        return None;
+    }
+    
+    String::from_utf8(buffer).ok()
+}