test(canvas): test_canvas_cancel_cross_user_rejected fails on unmodified origin/main #16549

Problem

tests/api/test_canvas.py::TestCanvasWebSocketStreaming::test_canvas_cancel_cross_user_rejected
fails on current origin/main, unrelated to any change in flight.

Evidence

Confirmed the test file is byte-identical to origin/main (no local edits):

$ git show origin/main:autobot-backend/tests/api/test_canvas.py > /tmp/orig_test_canvas.py
$ diff -q /tmp/orig_test_canvas.py autobot-backend/tests/api/test_canvas.py
(no output -- identical)

And it fails in isolation, not just in combination with other tests:

$ python3 -m pytest tests/api/test_canvas.py::TestCanvasWebSocketStreaming::test_canvas_cancel_cross_user_rejected -q
1 failed in 1.39s

Discovered via

Found while running a broader regression sweep (pytest tests/api -q, 556
passed) during #16529/#16530 (PR #16545) — this was the only failure, and
the diff in that PR does not touch canvas/websocket code at all.

Acceptance Criteria

• Root-cause why this test fails on an unmodified checkout (timing?
  fixture drift? a real regression merged without CI catching it?)
• Fix the test or the behavior it covers
• tests/api/test_canvas.py passes in full