fix(trace): add caller nodes to call graph for incoming edges

The GetCallGraph method was adding incoming edges to the root node
but not including the caller symbols in the graph's Nodes map,
causing incomplete graph results.

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

Author: yoanbernabeu

trace/store.go

@@ -361,6 +361,12 @@ func (s *GOBSymbolStore) GetCallGraph(ctx context.Context, symbolName string, de
 					graph.Edges = append(graph.Edges, edge)
 					edgeSeen[edgeKey] = true
 				}
+				// Ensure caller node is present in the graph.
+				if _, exists := graph.Nodes[edge.Caller]; !exists {
+					if syms, ok := s.index.Symbols[edge.Caller]; ok && len(syms) > 0 {
+						graph.Nodes[edge.Caller] = syms[0]
+					}
+				}
 			}
 		}
 	}