Add set_complete for type-checked completion assignment

[KapJI]

complete is not declared on argparse.Action, so add_argument(...).complete = shtab.FILE fails type checking with "Action" has no attribute "complete". Both examples/pathcomplete.py and examples/customcomplete.py suppress this with # mypy: disable-error-code="attr-defined".

set_complete(action, complete) makes the same assignment behind a typed signature. The attribute style is unchanged, so the examples keep it. I can convert them and remove the suppressions if you prefer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.87%. Comparing base (e1ab406) to head (0e16eee).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #259      +/-   ##
==========================================
+ Coverage   87.83%   87.87%   +0.03%     
==========================================
  Files           4        4              
  Lines         666      668       +2     
  Branches      138      138              
==========================================
+ Hits          585      587       +2     
  Misses         43       43              
  Partials       38       38              

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.