tests(cli/load): Add regression test for session killed during attach

[tony]

Summary

Adds a regression test documenting the fix for TmuxObjectDoesNotExist being raised when a session is killed while the user is attached via tmuxp load.

User Experience

After running tmuxp load, users would see this traceback printed to their terminal after detaching:

Traceback (most recent call last):
  File "/home/d/.local/bin/tmuxp", line 7, in <module>
    sys.exit(cli.cli())
  ...
  File ".../tmuxp/cli/load.py", line 152, in _load_attached
    builder.session.attach()
  File ".../libtmux/session.py", line 332, in attach
    self.refresh()
  File ".../libtmux/neo.py", line 242, in fetch_obj
    raise exc.TmuxObjectDoesNotExist(...)
libtmux.exc.TmuxObjectDoesNotExist: Could not find object

Root Cause

The issue was caused by libtmux's Session.attach() calling refresh() after attach-session returned. Since attach-session is a blocking interactive command, the session state can change arbitrarily during attachment - including being killed entirely.

Timeline

• Feb 2024: 9a5147aa - Session.attach() was added with the refresh() call
• Nov 2025: tmuxp fdafdd2b switched from attach_session() to attach()
• Users started experiencing the bug

Fix

The fix is in libtmux: tmux-python/libtmux#616

This PR adds a regression test to tmuxp to ensure the behavior is documented and tested.

Test plan

• Existing tests/cli/test_load.py tests pass (26 passed, 1 skipped)
• New regression test test_load_attached_handles_session_killed_during_attach passes

Related

• libtmux fix: fix(Session.attach): Remove refresh() call that fails after session killed libtmux#616

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.97%. Comparing base (06f5769) to head (b67ee0d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1002   +/-   ##
=======================================
  Coverage   72.97%   72.97%           
=======================================
  Files          26       26           
  Lines        1839     1839           
  Branches      347      347           
=======================================
  Hits         1342     1342           
  Misses        394      394           
  Partials      103      103           

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