fix(test): remove race-inducing idle tick from test data in test_detach_keeps_draining_into_buffer (#73854)

[kyssta-exe]

Problem

test_detach_keeps_draining_into_buffer uses FakeBridge([b"one", b"", b"two"]) where b"" is a literal idle tick between the two real chunks. In PtySession._drain(), a b"" chunk triggers await asyncio.sleep(0) followed by another run_in_executor round-trip before b"two can be consumed. The 50ms asyncio.sleep in the test is often insufficient for this extra executor hop — the test fails intermittently with assert replay == b"onetwo" where only b"one" has arrived.

Fix

Remove the b"" idle tick from the test data. The FakeBridge naturally returns b"" (idle) when its chunk list is exhausted, so removing the in-band idle tick doesn't reduce what the test validates — it still proves the drain continues after detach and accumulates data into the buffer.

Verification

pytest tests/test_pty_session.py -v  # 10/10 passed
# 20 consecutive runs of the specific test: 20/20 passed

Closes #73854

[teknium1]

Thanks @kyssta-exe — closing as resolved: test_detach_keeps_draining_into_buffer was removed entirely in today's suite-wide test prune (PR #74383), so the flake this fixed no longer exists. Your diagnosis of the race-inducing idle tick was correct and appreciated.