isotp: fix soft socket .select() drops ObjectPipe, causing sr1() to hang in threaded mode

The select() method was filtering out ObjectPipe instances (like the
sniffer's close_pipe) from its return value. This prevented the sniffer's
stop mechanism from working correctly in threaded mode - when sniffer.stop()
sent to close_pipe, the select() method would unblock but not return the
close_pipe, so the sniffer loop couldn't detect the stop signal and had to
rely on continue_sniff timing, causing hangs under load.

The fix includes close_pipe (ObjectPipe) instances in the select return
value, so the sniffer loop properly detects the stop signal via the
'if s is close_pipe: break' check.

Added two new tests:
- sr1 timeout with threaded=True (no response scenario)
- sr1 timeout with threaded=True and background CAN traffic

The new "ISOTPSoftSocket select returns control ObjectPipe" test directly
verifies that ISOTPSoftSocket.select() passes through ready ObjectPipe
instances (e.g. the sniffer's close_pipe). This test deterministically
FAILS without the fix and PASSES with it.

The integration tests (sr1 timeout with threaded=True) are kept for
end-to-end coverage but the race window is too narrow on Linux with
TestSocket to reliably trigger the bug.

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Ben Gardiner <ben.l.gardiner@gmail.com>

Author: Copilot