Server(fix[new_session]): Harden error handling and TMUX env restoration

[tony]

Summary

• new_session() deletes os.environ["TMUX"] before calling tmux but only restores it on the success path — any exception permanently leaks the deletion. This wraps all post-deletion code in try/finally.
• proc.stdout[0] raises an unhelpful IndexError if tmux produces no output. This adds an explicit guard with a descriptive LibTmuxException.

Split out from #576 — hardening that's independent of the race condition fix itself.

Changes

src/libtmux/server.py:

• Wrap all code after del os.environ["TMUX"] in try/finally to ensure restoration on any exception (including setup-phase errors like pathlib.Path.expanduser())
• Guard against empty proc.stdout with descriptive LibTmuxException

tests/test_server.py:

• test_new_session_empty_stdout — verifies error on empty stdout
• test_new_session_restores_tmux_env_on_error — verifies TMUX restored after cmd error
• test_new_session_restores_tmux_env_on_setup_error — verifies TMUX restored after setup-phase error

Test plan

• uv run pytest — 875 passed, 1 skipped
• uv run ruff check . — all checks passed
• uv run mypy src tests — no issues

[codecov]

Codecov Report

❌ Patch coverage is 82.75862% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.76%. Comparing base (9da19ba) to head (a74c602).

Files with missing lines	Patch %	Lines
src/libtmux/server.py	82.75%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #628      +/-   ##
==========================================
+ Coverage   45.39%   45.76%   +0.36%     
==========================================
  Files          22       22              
  Lines        2249     2253       +4     
  Branches      360      361       +1     
==========================================
+ Hits         1021     1031      +10     
+ Misses       1082     1077       -5     
+ Partials      146      145       -1     

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