refactor: Enforce agentic API design across mcp-memory handlers

- Replaced unconstrained JSON serialization (serde_json) with dense YAML (serde_yaml) to prevent context window bloat (ADR-0122).
- Replaced 404/empty array dead ends with token-efficient LLM fallback hints in router and meta/graph handlers.
- Ensured graceful error fallbacks instead of hard crashes.

server/src/handlers/graph.rs

@@ -791,7 +791,7 @@ impl McpTool for SearchNodesHandler {
                     }
                 }
             }
-            Ok::<String, crate::error::AppError>(serde_yaml::to_string(&matched_entities)?)
+            if matched_entities.is_empty() { Ok::<String, crate::error::AppError>(format!("No entities found matching query. Try using a different keyword or relaxing the search query.")) } else { Ok::<String, crate::error::AppError>(serde_yaml::to_string(&matched_entities)?) }
         })?;
         Ok(data)
     }
@@ -832,7 +832,7 @@ impl McpTool for OpenNodesHandler {
                     result.entities.insert(name, e);
                 }
             }
-            Ok::<String, crate::error::AppError>(serde_yaml::to_string(&result)?)
+            if result.entities.is_empty() { Ok::<String, crate::error::AppError>(format!("No matching nodes found.")) } else { Ok::<String, crate::error::AppError>(serde_yaml::to_string(&result)?) }
         })?;
         Ok(data)
     }