COMP: Silence nodiscard warnings for Future.get() in PoolMultiThreader

Cast Future.get() return values to void where the call is only used
for its side effect of waiting for thread completion and propagating
exceptions.

Fixes the following warnings:
```
ITK/Modules/Core/Common/src/itkPoolMultiThreader.cxx:148:55: warning:
  ignoring return value of function declared with 'nodiscard'
  attribute [-Wunused-result]
ITK/Modules/Core/Common/src/itkPoolMultiThreader.cxx:303:11: warning:
  ignoring return value of function declared with 'nodiscard'
  attribute [-Wunused-result]

This is for clang-22 compilers.
```

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

Author: hjmjohnson

Modules/Core/Common/src/itkPoolMultiThreader.cxx

@@ -145,7 +145,7 @@ PoolMultiThreader::SingleMethodExecute()
   // so now it waits for each of the other work units to finish
   for (threadLoop = 1; threadLoop < m_NumberOfWorkUnits; ++threadLoop)
   {
-    exceptionHandler.TryAndCatch([this, threadLoop] { m_ThreadInfoArray[threadLoop].Future.get(); });
+    exceptionHandler.TryAndCatch([this, threadLoop] { (void)m_ThreadInfoArray[threadLoop].Future.get(); });
   }
 
   exceptionHandler.RethrowFirstCaughtException();
@@ -300,7 +300,7 @@ PoolMultiThreader::ParallelizeImageRegion(unsigned int         dimension,
               filter->IncrementProgress(0);
             }
           } while (status != std::future_status::ready);
-          m_ThreadInfoArray[i].Future.get();
+          (void)m_ThreadInfoArray[i].Future.get();
           reporter.CompletedPixel();
         });
       }