fix(security): avoid exposing error details in ThreadManagementController

Replace e.getMessage() with generic error messages to prevent
leaking internal error information to external users.
Resolves code scanning alerts #49 and #50.

Author: Yuriy Bezsonov

apps/unicorn-store-spring/src/main/java/com/unicorn/store/controller/ThreadManagementController.java

@@ -46,7 +46,7 @@ private Result tryStartThreads(int count) {
             threadGeneratorService.startThreads(count);
             return new Success("Successfully started " + count + " threads");
         } catch (IllegalStateException e) {
-            return new Failure(e.getMessage());
+            return new Failure("Unable to start threads. Threads may already be running.");
         }
     }
 
@@ -55,7 +55,7 @@ private Result tryStopThreads() {
             threadGeneratorService.stopThreads();
             return new Success("Successfully stopped all threads");
         } catch (IllegalStateException e) {
-            return new Failure(e.getMessage());
+            return new Failure("Unable to stop threads. No active threads found.");
         }
     }