SumSpectra reserve EventList based on Event Type (#39586)

### Description of work
Similar to #39348, This PR aims to fix a performance issue where
EventList::operator+=() was being used inefficiently. This caused Mantid
to hang due to repeated memory reallocation as each EventList was
appended. This fix improves performance by correctly preallocating
memory based on the eventType of each EventList, ensuring the
appropriate storage structure is reserved up front.

Relates to
[EWM12012](https://ornlrse.clm.ibmcloud.com/ccm/web/projects/Neutron%20Data%20Project%20%28Change%20Management%29#action=com.ibm.team.workitem.viewWorkItem&id=12012)


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Author: searscr

Framework/Algorithms/src/SumSpectra.cpp

@@ -583,7 +583,7 @@ void SumSpectra::execEvent(const MatrixWorkspace_sptr &outputWorkspace, Progress
     }
     numOutputEvents += inputWorkspace->getSpectrum(i).getNumberEvents();
   }
-
+  outputEL.switchTo(inputWorkspace->getSpectrum(0).getEventType());
   outputEL.reserve(numOutputEvents);
 
   // Loop over spectra

docs/source/release/v6.13.0/Framework/Algorithms/Bugfixes/39586.rst

@@ -0,0 +1 @@
+- Fix an issue where :ref:`SumSpectra <algm-SumSpectra>` would freeze when on large datasets when adding EventList to the Output Workspace if the Event Type was weighted.