fix(cr-2026-04-22): MCP stdio replies with parse-error on malformed JSON

MAJOR from the 2026-04-22 review (crates/mcp/src/main.rs:26-30): the
stdio transport logged malformed JSON lines to stderr and continued
without sending any JSON-RPC response. Clients saw silence instead of
the -32700 Parse Error they could key off. handle_message has a
parse-error branch for shape mismatch, but it never ran for bytes
that failed to parse as JSON at all.

Exposes a new public helper kon_mcp::parse_error_response(detail)
that mirrors the existing internal error_response pattern, filling
id with null per JSON-RPC 2.0 §5.1 (parse error, no id recoverable).
main.rs now writes that response out before continuing the read
loop.

Regression test on the helper asserts: jsonrpc "2.0", id null,
code -32700, message starts with "Parse error" and includes the
underlying serde detail.

crates/mcp/src/lib.rs

@@ -335,6 +335,16 @@ fn error_response(id: Value, code: i32, message: String) -> JsonRpcResponse {
     }
 }
 
+/// Build a JSON-RPC 2.0 Parse Error response (code -32700, id null),
+/// for use by the stdio transport when a raw line fails to parse as
+/// JSON at all. `handle_message` covers the shape-mismatch case; this
+/// helper covers the `serde_json::from_str` failure in `main.rs` so
+/// clients receive a well-formed JSON-RPC reply instead of silence
+/// (2026-04-22 review MAJOR).
+pub fn parse_error_response(detail: &str) -> JsonRpcResponse {
+    error_response(Value::Null, -32700, format!("Parse error: {detail}"))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -398,6 +408,18 @@ mod tests {
         );
     }
 
+    #[test]
+    fn parse_error_response_has_jsonrpc_2_0_shape() {
+        let resp = parse_error_response("expected value at line 1 column 1");
+        assert_eq!(resp.jsonrpc, "2.0");
+        assert_eq!(resp.id, Value::Null);
+        assert!(resp.result.is_none());
+        let err = resp.error.expect("parse_error_response must carry an error");
+        assert_eq!(err.code, -32700);
+        assert!(err.message.contains("Parse error"));
+        assert!(err.message.contains("expected value"));
+    }
+
     #[tokio::test]
     async fn unknown_method_returns_method_not_found_error() {
         let request = json!({