🍒resultmodel: guard model in threaded resource cleanup

this previously was subject to a race condition between ResultModel destruction and the file access finishing. When the file access took long enough the Model would have been deleted leading to a crash.

Instead weakly point to the model via the QObject such that we can then check if still exists by the time we want to call into it

BUG: 458121 BUG: 466995 Fixes PLASMA-WORKSPACE-YZ

(cherry picked from commit d9d2c8b3)