fix: parse flat content arrays in Responses API input

When input is [{type: 'input_image', ...}, {type: 'input_text', text: '...'}],
the code was looking for items with role: 'user' which don't exist in flat
content arrays. Now extracts text from input_text items directly first,
falling back to role-based messages only if no flat text found.

Also adds debug header dump for MITM request forwarding.

src/mitm/proxy.rs

@@ -633,6 +633,16 @@ async fn handle_http_over_tls(
         };
 
         // Forward the request — if write fails, reconnect and retry once
+        // DEBUG: dump headers and total size
+        if req_path.contains("streamGenerateContent") {
+            let hdr_end = find_headers_end(&request_buf).unwrap_or(request_buf.len());
+            let hdr_str = String::from_utf8_lossy(&request_buf[..hdr_end.min(request_buf.len())]);
+            info!(
+                total_buf_len = request_buf.len(),
+                body_len = request_buf.len() - hdr_end,
+                "MITM: sending request to upstream\n{hdr_str}"
+            );
+        }
         if let Err(e) = conn.write_all(&request_buf).await {
             debug!(domain, error = %e, "MITM: upstream write failed, reconnecting");
             let c = connect_upstream(domain, &upstream_config).await?;