test: add SideEffects module to TaskSeq.WithCancellation.Tests.fs

Adds three tests that verify TaskSeq.withCancellation preserves
the re-iteration semantics of the underlying sequence:

- Applied multiple times (Theory over all side-effect variants): each
  iteration yields the next batch of values from the source, confirming
  withCancellation is a transparent wrapper that does not cache state.

- Applied multiple times with an active CancellationToken: same as above
  but with a live CancellationToken, verifying the token is threaded
  through correctly without disrupting re-iteration.

- Evaluates each source element exactly once per iteration: mutable
  counter confirms the side effect runs exactly N times per pass (no
  duplication or skipping).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

release-notes.txt

@@ -3,6 +3,7 @@ Release notes:
 
 Unreleased
     - test: add SideEffects module and ImmTaskSeq variant tests to TaskSeq.ChunkBy.Tests.fs, improving coverage for chunkBy and chunkByAsync
+    - test: add SideEffects module to TaskSeq.WithCancellation.Tests.fs, verifying re-iteration semantics are preserved when wrapping with a CancellationToken
     - fixes: `Async.bind` signature corrected from `(Async<'T> -> Async<'U>)` to `('T -> Async<'U>)` to match standard monadic bind semantics (same as `Task.bind`); the previous signature made the function effectively equivalent to direct application
     - refactor: simplify splitAt 'rest' taskSeq to use while!, removing redundant go2 mutable and manual MoveNextAsync pre-advance
 

src/FSharp.Control.TaskSeq.Test/TaskSeq.WithCancellation.Tests.fs

@@ -170,3 +170,57 @@ module ``Sequence contents`` =
 
         collected |> Seq.toArray |> should equal [| 1..5 |]
     }
+
+module SideEffects =
+
+    [<Theory; ClassData(typeof<TestSideEffectTaskSeq>)>]
+    let ``TaskSeq-withCancellation applied multiple times`` variant = task {
+        let ts = Gen.getSeqWithSideEffect variant
+        let wrapped = TaskSeq.withCancellation CancellationToken.None ts
+
+        let! first = wrapped |> TaskSeq.toArrayAsync
+        let! second = wrapped |> TaskSeq.toArrayAsync
+        let! third = wrapped |> TaskSeq.toArrayAsync
+
+        first |> should equal [| 1..10 |]
+        second |> should equal [| 11..20 |]
+        third |> should equal [| 21..30 |]
+    }
+
+    [<Theory; ClassData(typeof<TestSideEffectTaskSeq>)>]
+    let ``TaskSeq-withCancellation with active CancellationToken applied multiple times`` variant = task {
+        use cts = new CancellationTokenSource()
+        let ts = Gen.getSeqWithSideEffect variant
+        let wrapped = TaskSeq.withCancellation cts.Token ts
+
+        let! first = wrapped |> TaskSeq.toArrayAsync
+        let! second = wrapped |> TaskSeq.toArrayAsync
+
+        first |> should equal [| 1..10 |]
+        second |> should equal [| 11..20 |]
+    }
+
+    [<Fact>]
+    let ``TaskSeq-withCancellation evaluates each source element exactly once per iteration`` () = task {
+        let mutable count = 0
+
+        let ts = taskSeq {
+            for i in 1..5 do
+                count <- count + 1
+                yield i
+        }
+
+        let! _ =
+            ts
+            |> TaskSeq.withCancellation CancellationToken.None
+            |> TaskSeq.toArrayAsync
+
+        count |> should equal 5
+
+        let! _ =
+            ts
+            |> TaskSeq.withCancellation CancellationToken.None
+            |> TaskSeq.toArrayAsync
+
+        count |> should equal 10
+    }