Fix ISOTPSoftSocket.select() dropping ObjectPipe, causing sr1() to hang (fix #4838)

[BenGardiner]

( I worked this out with Copilot / Claude Opus 4.6 here: BenGardiner#1 to try to fix the ISO TP soft socket problem that has been plaguing me )

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 unit tests:

• sr1 timeout with threaded=True (no response scenario)
• sr1 timeout with threaded=True and background CAN traffic
• Add deterministic unit test that fails without fix; keep integration tests

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.

Verification:

• Without fix: "select returns control ObjectPipe" = FAILED
• With fix: "select returns control ObjectPipe" = PASSED
• All 67 test cases pass with the fix applied
• Manual Testing (on the same setup that causes the failure noted in Isotp soft socket sr1 hang/no return and timeout= was specified #4838, those table rows included for reference)

ver	driver	bg traffic	sr1() threaded=?	ISOTP DNE result	RDID SF req + MF resp result
2.6.0-rc2	candle	YES	False	timeouts ✅	data ✅
2.6.0-rc2	candle	YES	True	hang ❌	data ✅
2.6.0-rc2	slcan	YES	False	timeouts ✅	timeouts ❌
2.6.0-rc2	slcan	YES	True	hang ❌	timeouts [^1] ❌
2.6.0	candle	YES	False	timeouts ✅	data ✅
2.6.0	candle	YES	True	hang ❌	data ✅
2.6.0	slcan	YES	False	timeouts ✅	timeouts [^1] ❌
2.6.0	slcan	YES	True	hang ❌	hang ❌
this PR	candle	YES	False	timeouts ✅	data ✅
this PR	candle	YES	True	timeouts ✅	data ✅
this PR	slcan	YES	False	timeouts ✅	timeouts ❌
this PR	slcan	YES	True	timeouts ✅	timeouts ❌

This fixes #4838

The timeouts on slcan for a SF request -> MF response are a pre-existing problem.

NB: copilot/claude also suggested this optimization BenGardiner@8992e1b which seems correct to me but is not directly related to fixing the sr1() hang I've observed. It is not included in this PR.

cc: @polybassa

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.64%. Comparing base (2c787cd) to head (9ce1d81).
⚠️ Report is 15 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4929      +/-   ##
==========================================
+ Coverage   80.10%   80.64%   +0.53%     
==========================================
  Files         370      370              
  Lines       91733    91735       +2     
==========================================
+ Hits        73482    73976     +494     
+ Misses      18251    17759     -492     

Files with missing lines	Coverage Δ
scapy/contrib/isotp/isotp_soft_socket.py	85.24% <100.00%> (+0.05%)	⬆️

... and 20 files with indirect coverage changes

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