Fix flaky bufferByCountAndTime: reset window when buffer transitions from empty

When an item arrives into an empty buffer, the remaining time (rt - delta)
could be near-zero if the item arrived just before the timer fired. This
caused a Sleep(~0ms) timer in the next iteration, which could expire before
the next item arrived on a loaded CI machine.

Fix: when the first item enters the buffer (count goes 0→1), start a fresh
timeoutMs window. The timer window should measure how long items have been
accumulating, not track time from when the buffer was last cleared.

Fixes the flaky AsyncSeq.bufferByTimeAndCount test.

Co-authored-by: Copilot <[email protected]>

Author: github-actions[bot]

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -2277,7 +2277,10 @@ module AsyncSeq =
             buffer.Clear()
             yield! loop None timeoutMs
           else
-            yield! loop None (rt - delta)
+            // When the first item arrives into an empty buffer, start a fresh window.
+            // Otherwise, track remaining time in the current window.
+            let nextRt = if buffer.Count = 1 then timeoutMs else rt - delta
+            yield! loop None nextRt
         | Choice2Of2 (_, rest) ->
           if buffer.Count > 0 then
             yield buffer.ToArray()