fix(gateway): forward structuredContent in proxied tool results

Tools with output schemas (e.g. Google Workspace MCP) return structuredContent
alongside text content. The gateway was dropping this field when proxying
CallToolResult, causing clients to reject otherwise successful tool calls.

Signed-off-by: crimsonsunset <jsangio1@gmail.com>

Author: crimsonsunset

crates/mcpmux-gateway/src/mcp/handler.rs

@@ -852,11 +852,12 @@ impl ServerHandler for McpMuxGatewayHandler {
             "call_tool result"
         );
 
-        let result = if tool_result.is_error {
+        let mut result = if tool_result.is_error {
             CallToolResult::error(content)
         } else {
             CallToolResult::success(content)
         };
+        result.structured_content = tool_result.structured_content;
 
         Ok(result)
     }

crates/mcpmux-gateway/src/pool/routing.rs

@@ -51,6 +51,7 @@ pub struct RoutedResource {
 #[derive(Debug)]
 pub struct ToolCallResult {
     pub content: Vec<Value>,
+    pub structured_content: Option<Value>,
     pub is_error: bool,
 }
 
@@ -304,6 +305,7 @@ impl RoutingService {
 
                     Ok(ToolCallResult {
                         content,
+                        structured_content: res.structured_content,
                         is_error: res.is_error.unwrap_or(false),
                     })
                 }