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/meta.rs

@@ -126,7 +126,7 @@ impl McpTool for SearchErrorFixesHandler {
                     })
                     .collect();
 
-                Ok::<String, crate::error::AppError>(serde_yaml::to_string(&suggestions)?)
+                if suggestions.is_empty() { Ok::<String, crate::error::AppError>("No matching error fixes found. Try relaxing the search query.".to_string()) } else { Ok::<String, crate::error::AppError>(serde_yaml::to_string(&suggestions)?) }
             })?;
             return Ok(data);
         }
@@ -153,7 +153,7 @@ impl McpTool for SearchErrorFixesHandler {
                     }
                 })
                 .collect();
-            Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
+            if filtered.is_empty() { Ok::<String, crate::error::AppError>(format!("No matching results found.")) } else { Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?) }
         })?;
         Ok(data)
     }
@@ -339,7 +339,7 @@ impl McpTool for QueryRecentChangesHandler {
                     c
                 })
                 .collect();
-            Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
+            if filtered.is_empty() { Ok::<String, crate::error::AppError>(format!("No matching results found.")) } else { Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?) }
         })?;
         Ok(data)
     }
@@ -552,7 +552,7 @@ impl McpTool for DecisionsHandler {
                         .collect();
 
                     if include_body {
-                        Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
+                        if filtered.is_empty() { Ok::<String, crate::error::AppError>(format!("No matching results found.")) } else { Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?) }
                     } else {
                         let compact: Vec<_> = filtered
                             .iter()