Fix antitarget crash when --output is omitted (#806)

[etal]

Fixes #806.

Problem

cnvkit.py antitarget <targets> raised AttributeError: 'Namespace' object has no attribute 'interval' whenever -o/--output was omitted. The default-output-filename branch of _cmd_antitarget derived the name from args.interval, but the antitarget subparser defines its positional as targets and has no interval attribute. (interval is the positional of the separate target command; this was a copy-paste from _cmd_target.) Because the branch always failed on that attribute access, the default-output path never worked.

Fix

Derive the default name from args.targets (one token). The handler already reads args.targets when loading the input, so it is now internally consistent.

Test

Adds test/test_preprocessing.py::PreprocessingTests::test_cmd_antitarget_default_output, which exercises _cmd_antitarget with output=None and asserts the derived <base>.antitarget.bed is written and parses. This path was previously uncovered (the existing test_antitarget calls do_antitarget directly, never the CLI wrapper). Revert-verified: the test fails on the old args.interval line with the exact AttributeError and passes on the fix.

Note (out of scope)

The fix makes a previously-unreachable branch live. That branch's filename derivation uses str.rsplit('.', 1), which would raise ValueError on an extension-less targets filename; os.path.splitext would handle it gracefully. This is pre-existing and does not affect normal BED/interval inputs (which always carry an extension), so it is left as a possible follow-up rather than folded into this minimal fix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.75%. Comparing base (6acfbc7) to head (e85ee3b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1124      +/-   ##
==========================================
+ Coverage   72.68%   72.75%   +0.07%     
==========================================
  Files          74       74              
  Lines        8149     8149              
  Branches     1447     1447              
==========================================
+ Hits         5923     5929       +6     
+ Misses       1781     1774       -7     
- Partials      445      446       +1     

Flag	Coverage Δ
unittests	72.75% <100.00%> (+0.07%)	⬆️

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