Enable faulthandler and force OFF_SCREEN by default

[banesullivan]

Every downstream PyVista project sets faulthandler.enable() and pyvista.OFF_SCREEN = True on its own, and the ones that forget get opaque segfaults or hanging headless CI. The plugin now does both: faulthandler is enabled when the plugin is imported, and OFF_SCREEN is forced during configuration.

Behavior change: a project that intentionally runs on-screen under pytest must set pyvista_off_screen = false. faulthandler is enabled unconditionally at import with no opt-out.

resolves #276

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.81%. Comparing base (4129aa4) to head (af4a4ea).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
+ Coverage   95.79%   95.81%   +0.02%     
==========================================
  Files           4        4              
  Lines         998     1003       +5     
  Branches      131      132       +1     
==========================================
+ Hits          956      961       +5     
  Misses         30       30              
  Partials       12       12              

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