fix(mcp): return explicit JSON-RPC errors instead of misleading Ok strings for missing entities and silent graph modification drops

server/src/handlers/tasks.rs

@@ -121,7 +121,7 @@ impl McpTool for DeleteTaskHandler {
             ][0]
             .clone())
         } else {
-            Ok("Task not found.".to_string())
+            Err("Task not found.".to_string())
         }
     }
 }
@@ -264,7 +264,7 @@ impl McpTool for UpdateTaskStatusHandler {
         } else if found {
             Ok("Task status updated.".to_string())
         } else {
-            Ok("Task not found.".to_string())
+            Err("Task not found.".to_string())
         }
     }
 }
@@ -344,7 +344,7 @@ impl McpTool for SetAcceptanceCriteriaHandler {
         if success {
             Ok("Acceptance criteria set successfully.".to_string())
         } else {
-            Ok("Task not found.".to_string())
+            Err("Task not found.".to_string())
         }
     }
 }
@@ -393,7 +393,7 @@ impl McpTool for VerifyAcceptanceCriteriaHandler {
         } else if already_met {
             Ok("Acceptance criteria was already met.".to_string())
         } else {
-            Ok("Acceptance criteria or task not found.".to_string())
+            Err("Acceptance criteria or task not found.".to_string())
         }
     }
 }
@@ -452,7 +452,7 @@ impl McpTool for UpdateMilestoneHandler {
         if found {
             Ok("Milestone updated".to_string())
         } else {
-            Ok("Milestone not found".to_string())
+            Err("Milestone not found".to_string())
         }
     }
 }