feat(mcp): switch to YAML serialization for tool outputs

- Replaced serde_json with serde_yaml in all handler endpoints.

- This drastically reduces token consumption for large list/graph responses by removing JSON braces and quotes.

server/src/error.rs

@@ -40,6 +40,9 @@ pub enum AppError {
 
     #[error("I/O Error: {0}")]
     IoError(#[from] std::io::Error),
+
+    #[error("YAML Serialization Error: {0}")]
+    YamlError(#[from] serde_yaml::Error),
 }
 
 impl From<AppError> for String {
@@ -81,6 +84,10 @@ impl IntoResponse for AppError {
                 StatusCode::INTERNAL_SERVER_ERROR,
                 format!("I/O Error: {}", e),
             ),
+            AppError::YamlError(e) => (
+                StatusCode::INTERNAL_SERVER_ERROR,
+                format!("YAML Serialization Error: {}", e),
+            ),
         };
 
         let body = Json(json!({