fix: propagate parent cancel token to parallel tactic subtasks (#13428)

This PR fixes parallel tactic combinators (`attempt_all_par`,
`first_par`) leaking their subtasks when the server cancels elaboration
on re-elaboration. Subtasks spawned via `CoreM.asTask` (and its
`MetaM`/`TermElabM`/`TacticM` variants) get a fresh `IO.CancelToken`,
which previously had no link to the parent token; `cancelRec` would set
the command-level token but the children kept running.

The fix is one line in `CoreM.asTask`: when a parent token is in scope,
register `cancelToken.set` as an `onSet` callback on the parent.
Server-level cancellation now flows down to every parallel subtask, and
`Core.checkInterrupted` inside the child sees the token set as expected.

Fixes #13300.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/Lean/Elab/Task.lean

@@ -51,6 +51,11 @@ def asTask (t : CoreM α) : CoreM (BaseIO Unit × Task (CoreM α)) := do
   -- but modify it to return both the result and state
   let wrappedAct ← Core.wrapAsync (fun () => do let a ← t; let s ← get; return (a, s)) (some cancelToken)
   let task ← (wrappedAct ()).asTask
+  -- Propagate parent cancel token to child: when the parent is cancelled,
+  -- the child token is set too. This ensures that subtasks spawned via `asTask`
+  -- respond to server-level cancellation (e.g. on re-elaboration).
+  if let some parentTk := (← read).cancelTk? then
+    parentTk.onSet cancelToken.set
   return (cancelToken.set, task.map (sync := true) fun result =>
     match result with
     | .ok (a, s) => do

tests/server_interactive/cancellation_par.lean.out.expected

@@ -2,7 +2,5 @@ blocked!
 cancelled!
 blocked!
 cancelled!
-attempt_all_par: leaked!
 blocked!
 cancelled!
-first_par: leaked!