[problems-view] IJPL-250021 add guard for file validity during quick-fix retrieval

problem events get emitted to a flow, and are then processed into dtos and sent to the fe. if the file corresponding to the problem gets deleted during the (possibly slow) processing of the events, there is a small window between:
 - the file getting deleted, and
 - the cancellation of the coroutine scope associated with that file

 in which we try to get the `PsiFile` associated with it, and that is an invalid operation

GitOrigin-RevId: 224594f181164b1addbb1498cbed9cf0810ab6e3

platform/problemsView/backend/src/com/intellij/platform/problemsView/backend/ProblemDtoBuilders.kt

@@ -123,6 +123,8 @@ internal suspend fun buildProblemDto(problem: Problem, lifetime: ProblemLifetime
 }
 
 private suspend fun getQuickFixesOfProblem(problem: HighlightingProblem): List<IntentionActionWithOptions> = readAction {
+  if (!problem.file.isValid) return@readAction emptyList()
+
   val psiFile = PsiManager.getInstance(problem.provider.project).findFile(problem.file)
   if (psiFile == null) return@readAction emptyList()