Fix bug 5: truncate coverage error output to last 5 lines

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/http/workflow.rs

@@ -452,7 +452,25 @@ impl WorkflowApi {
 
         if !output.status.success() {
             let stderr = String::from_utf8_lossy(&output.stderr);
-            return Err(bad_request(format!("Coverage command failed: {stderr}")));
+            let stdout = String::from_utf8_lossy(&output.stdout);
+            let combined: Vec<&str> = stdout
+                .lines()
+                .chain(stderr.lines())
+                .filter(|l| !l.trim().is_empty())
+                .collect();
+            let tail: Vec<&str> = combined
+                .iter()
+                .rev()
+                .take(5)
+                .rev()
+                .copied()
+                .collect();
+            let summary = if tail.is_empty() {
+                "Unknown error. Check server logs for details.".to_string()
+            } else {
+                tail.join("\n")
+            };
+            return Err(bad_request(format!("Coverage command failed:\n{summary}")));
         }
 
         // Read the coverage summary JSON