fix: make IO-bound tasks dedicated (#5678)

This PR ensures that all I/O-bound tasks in the language server use
dedicated tasks.

src/Lean/Server/FileWorker.lean

@@ -569,7 +569,7 @@ section MessageHandling
     let text := st.doc.meta.text
 
     match st.importCachingTask? with
-    | none => IO.asTask do
+    | none => IO.asTask (prio := Task.Priority.dedicated) do
       let availableImports ← ImportCompletion.collectAvailableImports
       let lastRequestTimestampMs ← IO.monoMsNow
       let completions := ImportCompletion.find text st.doc.initSnap.stx params availableImports
@@ -697,7 +697,7 @@ end MainLoop
 
 def runRefreshTask : WorkerM (Task (Except IO.Error Unit)) := do
   let ctx ← read
-  IO.asTask do
+  IO.asTask (prio := Task.Priority.dedicated) do
     while ! (←IO.checkCanceled) do
       let pastProcessingStates ← ctx.chanIsProcessing.recvAllCurrent
       if pastProcessingStates.isEmpty then

src/Lean/Server/Watchdog.lean

@@ -995,7 +995,7 @@ section MainLoop
       /- Runs asynchronously. -/
       let msg ← st.hIn.readLspMessage
       pure <| ServerEvent.clientMsg msg
-    let clientTask := (← IO.asTask readMsgAction).map fun
+    let clientTask := (← IO.asTask (prio := Task.Priority.dedicated) readMsgAction).map fun
       | Except.ok ev   => ev
       | Except.error e => ServerEvent.clientError e
     return clientTask
@@ -1161,7 +1161,7 @@ results in requests that need references.
 def startLoadingReferences (references : IO.Ref References) : IO Unit := do
   -- Discard the task; there isn't much we can do about this failing,
   -- but we should try to continue server operations regardless
-  let _ ← IO.asTask do
+  let _ ← IO.asTask (prio := Task.Priority.dedicated) do
     let oleanSearchPath ← Lean.searchPathRef.get
     for path in ← oleanSearchPath.findAllWithExt "ilean" do
       try